Homeowners who are illegally renting out flats to holidaymakers in Spain are being caught by police because of the large number of towels hanging over balconies.
Tourist hot spots across the country are cracking down on "black market" rentals as private owners often don't pay tax and are seen as unfair competition to hotels.
Under sweeping new rules, owners of flats and villas who want to rent out their homes must gain a licence from the local council and declare everyone who stays in them and the money paid. If not, they will face hefty fines.
In the Balearics, other residents of apartment blocks who live in their flats permanently have been told to shop anyone suspected of illegal holiday rentals.
And whistle-blowers are said to be turning up at the council offices armed with information and addresses of illegal lettings and even copies of advertisements.
Many have been alerted because of noise and parties on balconies or terraces after midnight or by the vast amount of beach towels hanging over the railings.
In Ibiza during July, the owners of 28 flats were officially reported and this number is expected to snowball as more inspectors are brought in to join the police.
These particular properties were in the popular areas of Vila, Sant Joseph and Sant Antoni.
In one single block in Platja d'en Bossa in Ibiza, 22 flats were found to be rented out to holidaymakers illegally.
In another block in Sant Joseph, residents said there were at least six flats being rented out regularly to tourists.
Inspectors have warned that even though no-one was in at other properties visited, they will be returning for further spot checks.
The situation is being reflected across Spain where all owners renting out properties for holidays are being warned to follow the new rules.
Owners of illegal lettings will face fines of up to 40,000 euros ($70,000) with substantial reductions for early payment.
After this, they would have to withdraw them from the market or face further penalties.