Is it possible to pinpoint all the BSSIDs that exist in a building? For instance in the case of other buildings nearby a scan would also find networks of neighboring buildings. Is there a strategy to overcome that?
"Wireless Heat Mapping" is the usual term. Various software exists for doing it; I linked to a free one I'm not in any way associated with. A decent job requires access to the whole building, so if you are merely trolling hallways the results are likely to be much less definitive.
A signal from a neighboring building will be generally broad and weak, while signals from within the building will have zones of higher signal that (seen on a heatmap) will indicate where in the building they are coming from.