Gravity strikes

Something metal fell out of the ceiling a moment ago. Fortunately, no-one was seated below it.

We have something not unlike moles in the office today, however instead of leaving small piles of dirt in a line across the carpet they are knocking out ceiling tiles, making electric drill and screwdriver noises, and dropping angle brackets. I don't know what they're doing up there because you never see anyone, you just turn round and there's another mysterious black hole opened up in the ceiling, with wires dimly visible through.

Presumably there's some kind of workmens' access between the ceiling of this storey and the floor of the one above (unless we have semi-evolved rats who've reached the tool-using stage of civilisation, which wouldn't surprise me). It's an odd thought. Floors is floors and roofs is roofs and the idea that there's a whole different system of moving around the building sandwiched between them is rather strange.

What sort of people might inhabit this netherworld? Why do we never see them? Should we leave food out for them? And what are they going to drop on our heads next?