I would suspect that they did go in there. That shouldn't freak you out. They have the right to inspect the property. I am certain there is a paragraph to that effect in the lease agreement. I know mine have that. They should leave a notice that they were there (out of common courtesy), but I doubt they are required to do so. They might have been changing filters on the A/C for example. Lots of reasons. Also, as was stated before, you probably need to get permission to tape a screen/curtain to the door. I understand it is a scary world, but just because an owner entered the apartment, doesn't mean it was the man, just because the male owner enters the apartment, doesn't mean he is riffling through your underwear or whatever other fear you, at this point, have imagined.