They were heading back to their Te Poi home when he jumped out the open window at the back of their canopied ute.
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"We've got home and not realised and he's just not there," said Mrs Keir, who is 8-months pregnant.
"He's travelled that way his whole life. It's just so out of character for him to do that."
The couple tracked down CCTV footage from along the journey which shows he was in the ute at the Elizabeth St roundabout but was gone by the time they got to the toll gate on Route K.
Mrs Keir said both she and her husband were self-employed so had put work on hold and spent every day searching for their dog.
Mr Keir, his friend and his mother spent eight hours searching the bush along the side of Route K in the hope he would still be in the area they lost him.
They have plastered hundreds of posters around the city, taken to the radio, spoken to vets, been to the SPCA and set up Facebook page Bring Winston Home.
They have had two sightings so far, one in Yatton Park and then one on Thursday night in Oropi.
The couple have since searched those areas with no sign of Winston.
He has a distinctive black spot on his nose, a white tipped tail and is extremely friendly, Mrs Keir said. He was not wearing a collar but is micro-chipped.
If you have seen Winston or know where he is, contact Wendy on 027 469 7377.