I live in a apartment building where the ways are very thin.
The way I found out was when my next door neighbor was having sex and I heard everything. I was lying in my bed one night when I heard a moaning. I looked around the room and didn't know where it was coming from. Finally I was next to the wall I heard her say she was cumming. I laughed and laid back down. The next day when I saw her I asked if she had a good night. She told me yes and asked why. I just told her some lie and she left. That night I heard a vibrating noise coming from the other side of the wall. I got excited and decided to play with myself. I lotioned up my hands and started to stroke myself. I heard her moaning and talking and I started to stroke faster and faster. I came and made such a loud noise that the vibrating noise stopped. When I was her the next day she asked me how was my night and winked at me.