It seems innocuous enough, right? Abandoned building. Couldn't possibly be anyone in there.

Yeah, that's kind of what he's hoping, at least at first.
That there's no reason what so ever to come across the sagging and creaking threshold, let alone find your way down to the cellar.

Which doesn't really explain the trip wires, caltrops, or buckets of water over doorways.

Inside Mike is huddled around a small cobbled together terracotta space heater.
Donnie would be so pround.