My son was talking to a girl, and then she just disappeared

I'm a 26 year old father to a 5 year old little boy.<br>There is a little girl who lives next door to us.<br>She does not get visitors, so she must live here with her mother.<br>She is never outside on her own.<br>She can't go to school with kids yet.<br>So one day, my son is talking to her upstairs in the hallway, then one second she's there, the next second she's gone.<br>TOO weird. maybe I'm going crazy.<br>There has been a few other weird occurrences in this building.<br>But one time my son went to one room in our apartment, and I looked outside into the hallway, one second I didn't see any upstairs hallway, then I looked again and it suddenly appeared.