"Neighboring" as an adjective means "next-to". So it makes sense to talk about "the neighboring buildings". And we would understand "signal the neighboring buildings" as meaning "signal the people in the neighboring buildings".
The neighbor buildings is not used.
You can say "the neighborhood buildings" These are building that are close-by but not necessarily next-to the building where the man is.
You can say "the neighbor's buildings" these are the building owned by the neighbor. But that is rather odd, as it would seem to mean the buildings owned by the neighbor, whereever they might be. Apartment blocks are not usually owned by the people that live in them, so they aren't "the neighbor's"
All in all "the neighboring buildings" is the best option.