Maintenance in apartment complexes always have access to any apartment, in case of fire, overflowing bathtubs, inspections, deaths, smoke detector battery replacement, etc. They are supposed to give 24 hours notice, but the point is that a nefarious character could gain access to any apartment in the complex, if they don't keep their master keys secure.
We had a case recently of a murder in a gated complex. A maintenance guy got obsessed by the 19 year old daughter of a resident, and eventually kidnapped, raped, and murdered her. All because he had access to the master keys. They ended up passing some law under her name. I think they have to do a better job of clearing their criminal backgrounds, which would have caught this guy. It seems like keeping the master keys under better security should be a major thing, too.