OK, that's it. I did not buy a vacuum cleaner. Is there anything else to do during cruising?
I was able to suck in two ghosts on the street, and now I also saw a ghost in front of a building, positioned the trap, and tried to catch it with the trap, but failed, of course. Need more practice.
Funnily, I could not make sense of the map at first because I had this "Necker cube" illusion where you misconstruct an ambiguous shape; that is, I did not see the roof top on the upper right corner, but something like an atrium with walls on the lower left.
I remember I played Ghostbusters on the C64 of a friend back in the 80s; this really brings back memories.