Yidney

Dead center in the rear bumper cover held on by only one screw is a sensor that is about 4" long and 3/4" wide that is a black plastic tray filled with that shiny epoxy looking stuff. What is that? It seems to be held on too flimsily to be any sort of impact sensor.

Rudeney

It's the wallet sensor, used to detect excess cash that needs to be paid to your friendly local dealership for unscheduled maintenance.

No, seriously, I am pretty sure that's antenna for KeylessGo.