Posing on the roof of London's Millennium Dome, they look like any other young couple enjoying a big night out. It's a Saturday evening in September, and the duo are five months into a whirlwind romance.

Having met via a dating website, they had formalised their relationship in June, in a series of images posted on the photo-sharing network Instagram. One showed them cuddling in bed, with the 23-year-old man (who'd just departed for a fortnight's summer holiday) playfully grabbing his scantily-clad, 21-year-old companion by the neck.

"My man and I having cuddles before he left," she wrote.

"It's going to be a tough two weeks without him. I love you baby xxx."

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Another image showed the lovers holding hands, fingers intertwined, while the young man's pet puppy, a black cross-breed called Louell, rested his chin on her naked thigh: '#mummydaddyandbaby #familylife #myboys' read her caption.

Yet a long shadow has now been cast over these scenes of domestic bliss.

For the man in the pictures is Corrie McKeague, the RAF serviceman at the centre of perhaps the most bizarre manhunt in recent years.

He disappeared six days after the romantic weekend break in London, during a night out in Bury St Edmunds, Suffolk, in which he wore the same pink shirt and white jeans. He drove there in his blue BMW convertible, which he left in a pay-and-display car park.

The night saw him visit two bars, before being asked to leave a nightclub by bouncers because he'd 'consumed enough alcohol'.

He then visited a takeaway, before falling asleep for two hours in a shop doorway. Then, at 3.25am, he was recorded on CCTV film walking into a loading bay behind a branch of Gregg's.

Soon afterwards, in a town dotted with security cameras, McKeague vanished. He's not been seen since.

Detectives are baffled, saying the chances of him managing to slip out of the centre of the cathedral town without being caught on camera are as low as one or two per cent.

A huge PR campaign, run by his family, and with thousands of supporters on social media, has failed to shed any light on the mystery. Adding to the intrigue, meanwhile, are recent revelations about McKeague's relationship with his 21-year-old lover, April Oliver.

Corrie McKeague, 23, vanished after a night out in September and has not been seen since. Photo / Supplied
Corrie McKeague, 23, vanished after a night out in September and has not been seen since. Photo / Supplied

Despite extensive media coverage of McKeague's disappearance, her existence was kept quiet for months.

Her profile was mysteriously removed from the RAF man's social media accounts, while his family described him as 'happily single'.

That all suddenly changed on January 10, when Oliver gave a TV interview making the shock announcement that they were not just a couple, but that she was pregnant with his child.

The plot then thickened considerably on Thursday when it also emerged that both Oliver and McKeague had for some time been living an astonishing double life, trawling the internet looking for casual sex with strangers.

McKeague had been active on a website called Fab Swingers since 2014, when he'd created a profile saying he wanted to arrange anonymous liaisons with straight and lesbian couples and single women aged between 18 and 60.

The site listed among his interests 'blindfolds', 'making videos', 'group sex' and 'SM [sadomasochism]'.

He then set up a second, joint account with Oliver in August, declaring, "We are a young couple but we know what we want' and listing a string of depraved sex acts they wished to carry out with random men and women. Their profile carried a photo of a woman wearing lingerie.

Its existence was reported in the Mail after the RAF man's mother, Nicola Urquhart, revealed details in a statement on a Facebook page dedicated to his disappearance called 'Find Corrie'.

Around the same time, McKeague's brother, Darroch, described the 'dating sites' as 'just irrelevant' to his disappearance.

However, the police appear to take a different view, saying that McKeague's use of Fab Swingers is now 'part of our investigation'. It emerged yesterday that they are interviewing contacts the couple made on the site.

That is not surprising, given the inherently dangerous nature of the sexual encounters that McKeague appears to have pursued.

A typical one is chronicled in an endorsement carried on his page on Fab Swingers, which is illustrated with a topless photo of the RAF man next to the caption 'hello all, come say hi'. Dated April 15, 2016 (when he was supposedly dating Oliver), it was written by a 28-year-old woman from Cambridge using the moniker 'pinupchick88' who describes herself as 'quite brutal'.

She recalled their encounter by saying that McKeague 'turned up looking and smelling amazing, didn't take long for me to take his clothes off, and he didn't disappoint!'

McKeague then posted a review on her page saying "met this lovely lady yesterday, she was straight down to business" before graphically describing a series of unconventional sex practices.

Police are now investigating about 20 other 'friends' whom he knew via the website, including eight women, four transsexuals or transvestites, two men and six couples. Of particular interest is believed to be 'Oriental-transvestite', a user of Fab Swingers who claims to live in East London, not far from the Millennium Dome.

McKeague's unconventional sex life has prompted speculation that his disappearance could be linked to a casual liaison on the night he vanished. Such an encounter could either have occurred in Bury St Edmunds, or somewhere he travelled after possibly being picked up in a car.

His mobile telephone was traced to a spot near Barton Mills, a village 12 miles north-west of the town (and a well-known location where strangers meet for alfresco sex), in the early hours of that morning.

It was moving at a speed consistent to being in a vehicle, though stopped transmitting at around 8am, suggesting that it either ran out of batteries or was destroyed.

Detectives are also looking at claims that McKeague was active on Grindr, an app used by gay men for casual hook-ups. It made headlines during last year's trial of serial killer Stephen Port, who drugged and murdered four men he met on the site.

In October, a month after his disappearance, a member of the public exchanged messages with a user of Grindr called 'BiLad21' who had what appeared to be a previously unpublished image of McKeague as his profile picture.

Screenshots of the account, set up two years earlier, were reported to the police shortly afterwards. It remains unclear if the picture or the page was genuine.

McKeague's family deny speculation that he might be gay, and his Fab Swingers profile described him as 'straight'.

The date of the alleged Grindr sighting has nonetheless prompted another explanation for McKeague's disappearance: that he might be alive and well, having decided for some reason to deliberately vanish.

On September 28, just three days after going missing, his uncle, Tony Wringe, pleaded for information on Fill Your Boots, a Facebook chatroom for the Armed Forces.

It read: 'Corrie is my nephew and the family are genuinely worried now. He was out on the p*** in Bury on Friday night and hasn't been seen since. He's been suffering with depression recently - so who knows . . .'

The post was shared by 1,600 people. However, it was deleted from the site a couple of months later, without any explanation.

Military sources confirm that McKeague, who joined the RAF in October 2013 and served as a gunner in the RAF Regiment, had been given time off to be treated for mental health problems. Yet they say that treatment took place some time ago and he was 'jovial as far as we are aware'.

McKeague's mother, meanwhile, has denied her son was at all emotionally troubled, writing on Facebook this week: 'Quite a few people have asked if Corrie was suffering from depression and or mental health issues. NO he wasn't.'

That post has also since been deleted.

This curious episode highlights another strange feature of the manhunt - just how many of the public statements made since McKeague vanished appear to have subsequently been either contradicted or quietly removed.

Take, for example, whether he might have come to grief while attempting to drunkenly walk home to RAF Honington, the base where he lived about eight miles away. Initially, this was seen as the most credible explanation for his disappearance.

On September 28, Suffolk police said: 'He's known to go on nights out with his friends and make his way back to wherever he's based on foot. Our belief is that he's attempted to walk from Bury to RAF Honington.' Five days later, they repeated this line.

McKeague's mother initially appeared to agree. In mid-October, she was reported to have said: 'He did not regularly walk the long distance back to his base, but had done so in the past.'

However, on November 8, she posted on Facebook: 'Corrie has NEVER walked back from Bury to Honington.'

Similar apparent inconsistencies - assuming she was accurately quoted by the newspaper - are also evident in public comments she's made regarding her son's love life.

In October, she described him as 'very happily single', but then reacted to reports of his relationship with April Oliver in another (subsequently deleted) Facebook post, saying they were in a casual relationship: 'Corrie and April were on dating sites, they are young and were enjoying themselves. They were both seeing other people, this has not been hidden.'

Quite how those remarks can be squared with Oliver's varied social media posts (which describe McKeague as her 'boyfriend' and date back to June) is anyone's guess.

This week, McKeague's mother also wrote a post on Facebook (again later deleted) discussing Oliver's decision to go public about her pregnancy.

"Last week, April had to publicly tell the world about her private life, something we all tried to keep private for both her and their baby, predominately at April's insistence . . . Sadly others would not leave her alone . . . She was really left with no choice as it was coming out anyway."

Again, that version of events seems at odds with other comments.

For on December 21, when McKeague's mother said that the intention was to keep Oliver's pregnancy 'private', the latter had publicly announced it on Instagram, with a picture of her belly along with the caption: 'Little blob I love you xxx.'

Meanwhile, there have been a series of fallings-out between McKeague's family and Suffolk police. They stretch back to early December, when his mother (who works as a police liaison officer in her native Scotland) told the BBC that officers were failing to investigate his disappearance properly.

The family has since raised more than £50,000 via a crowd-funding website to pay private investigators to look into the case.

They are believed to now be scouring McKeague's social media accounts, plus his profiles on dating sites. It emerged on Wednesday that his Fab Swingers page had been active last weekend, leading to speculation that he was alive - only for police to establish that the account was in fact being accessed by his family.

Of course, the disappearance of a loved one is understandably stressful, and some erratic behaviour is perhaps to be expected by those close to McKeague.

Equally, his family can be forgiven for attempting to protect his privacy. But five months after he vanished, there remain important unanswered questions.

Why, for example, have none of the friends who, it was said, he was out with on the night he vanished been named or spoken publicly?

Why have no RAF colleagues discussed their missing comrade, or made pleas for information?

Why, if he intended to go on a drunken late night out, did McKeague park his BMW in a pay-and-display car park where he'd have to collect it by 9am the following day - rather than find a free parking space a short walk away?

Above all, why are so many astonishing revelations continuing to emerge?

For with every new detail we uncover, the less the disappearance of Corrie McKeague appears to make sense.