haha i live in an apartment building and i have always presumed they are sensors for lights to come on in the corridors, there is one out side every apartment door, so i just took off the one outside my door and it has a small grey cable in it. inside and in my water tank cupboard is my water meter with a small grey cable going into it, the meter says “smart meter” on it so i think the water company come round and scan the black round thing and it gives them a meter reading, being so close to your gas meter i guess its for gas meter readings. so they dont have to come in your house or you have to ring them up with it.