Big Brother May Be Watching

Noticed these conspicuous black discs outside the lift on the ground floor of my HDB block, affixed on the wall with a single screw. They've also been spotted at other neighbourhoods as well. Are these security markers? Transponders? RFID detectors? or devices attempting to help big brother monitor our movements in and out of blocks? It certainly got me intrigued, but I'm sure there's some banal explanation for it. It's certainly of no value, otherwise vandals would have removed it by now.