I have similar thoughts. While we do not know which neighbors heard what, if they did hear something, they are probably close neighbors and yet did not call. It makes me wonder why? You could be the guilty party? When LE asks what you heard because you live nearby, you give an account of what you were home and heard...
Then, however, I go back to the fact she did not generally go home at lunch, typically anyhow.
That is a lot of apartments and I read at the site they also do short term, like even three months? It is also hard to know in a huge apartment complex who might have moved who in or let them stay there, who they have visiting, etc.
I agree, I have thought of someone simply slipping back into their own apartment...
I also have wondered if she had reason or concern to think someone had been in her apartment at times? Not unheard of either. Think of even like your local panty thief? Someone installing a camera on a young single woman? Someone letting themselves in while you are gone because you have cable and they don't? I mean there are all sorts out there these days. You might go home on an off day and time to try to catch someone if you have had suspicions?
If she was called and drawn home that way, much of this speculation does not fit but we just don't know.
We know now there is an exterior entry, is there any interior entry to these apartments as well? I don't mean the balcony but an interior hallway door?
Again, just talking out loud and speculating. Jmo.