People watch their perimeters but not their overheads. There was a theft of a bunch of guns and other stuff from a now-defunct gun store hereabouts a couple of years ago. The thieves got in through the roof. Any number of times I've said "Hello" from my second floor balcony at someone on the ground below. They look around and look around, and I have to say, "Up here, hi" for them to look up at me.
When I was living with Wife2 in an apartment we heard of an apartment intrusion / break-in from the ceiling hatch to the attic of the building, which allowed access to all the apartments throughout the building.
I'm now on the second floor with the attic above me and it's darned hard to see up there, but there may be some kind of netting between the apartments. Ne'ertheless, I rigged a couple of noisy tin cans on my attic hatch as a kind of alarm and indicator of attempted entry.
Paranoid? You betcha sweet bippy I am. Lifelong experience has taught me, as I have often said, "Paranoia is a survival trait." I don't mean clinical paranoia, but alley-cat street-smart paranoia.
"Up here, Kirk, hello."
"Oh, hi. Didn't see you up there."
(Of course not.)
Terry
REF (Kind of a thin article on it, but here):
https://www.denverpost.com/2013/02/28/burglary-at-gunsmoke-gun-shop-in-wheat-ridge-reamins-a-mystery/