Could be a case of mistaken address on the part of the person/people that were there. Perhaps he/she/they thought someone else lived there. Could be a variety of things. But, seriously, at least put in a call to the police and perhaps they can send a patrol car through your complex tonight on the off chance you get a return visit. Don't call 911, but call the station directly and file a report or ask to have an officer drive through late tonight. Better safe than sorry.