Any one know what this could be? It looks almost like a door or window trim but I don't see anything missing on my truck. My wife found it after I left. So strange.
Mystery solved. It goes at the front of the rear doors. No idea how this fell off, must have never been installed properly as it fits in a channel. Just happy it fell out at my house.
Here is a shot of where it was missing and with it reinstalled.
A couple dots of 3m Weatherstrip and gasket adhesive should keep it in place. While you have it out, also put some in on the fuel door cover; which also seems to like to fall off.
It never dries totally hard and stays a little gooey, so you can still remove everything later and for trim pieces and weatherstripping that is supposed to be held in by friction, you don't need to use very much at all and a small tube should last seemingly forever.