Found in the back of my 2014 CX5. I've looked all over the car and can't figure out where it belongs. It looks like it should fit into a hole of the same shape. Anyone know what it is?

 

Looks like it might be for the rear latch in the trunk/cargo area where the hatch closes onto. It should just click in.

 

Alexmed2002 is correct.
The part number is shown on the picture you posted.
Liftgate Trim Scuff Plate Cap Part Number: KD4568894
If you open the lift gate and look down where in closes in the middle, you'll see the metal u shaped bar that the hatch latch locks onto.
It will be missing the that cover, just snap it in.

 

Bingo!

 

Correct! The one place I didn't look (closely enough, that is)