HDB officers were on the lookout for such flats at a 16-storey rental block with two- and three-room units in Circuit Road as part of their stepped-up checks last Tuesday.

Such checks are in addition to HDB's regular visits to all rental flats to ensure tenants are not illegally subletting the flats for profit.

Those with nothing to hide usually leave windows and doors open for ventilation as rental flats typically do not have air-conditioning, said the officers.

When The Straits Times joined them on their inspection last week, the officers spent little time at several flats that had their doors open, taking only a few seconds to verify the status of the tenants.

Equipped with a file detailing tenants' names and particulars, they went around in pairs, knocking on doors. They said they typically visit after working hours or during weekends, when tenants are likely to be at home.

Read the full story in Tuesday's edition of The Straits Times.