Back Bumper

I'm trying to figure out what could've caused this to happen to my bumper. It's on the top of the bumper.

I backed up into another car a few weeks back, but not in that spot. It was hit about a foot to the left of that spot. It was a very very light bump, like it barely made a sound. When we looked at our cars, there was not a scratch or scuff on either of our cars. I looked at my car after a day or two and there was still nothing there. Not sure if this bump had anything to do with it but just thought I'd inform you on it.