You don't always get paint loss though, it has happened to me before and the other persons door didn't have a mark on it.
It definatley happened when I was parked infield, as I said it wasn't there in the morning and I only noticed it when I went back to my car in the afternoon by which time your car had gone so I could not ask you about it, and the reason I posted it up was because obviously you were the only car that was parked next to me and the dent is consistent with one from someones door. Don't get me wrong i'm not trying to blame you, but hopefully you can see where I am coming from and I just wanted to see if you knew anything, as I know some people don't like to admit to things.
I will try again with the pic in the morning, but it is hard to pick it up on my phone, it is very noticable though.