Orange plastic sliding cover in lower cargo compartment?

It's plastic, it's orange and it's above the spare tire, to starboard, on the forward bulkhead. What is it?

There's a finger-hole, and you can slide the cover part to the left. I stoped playing with it at that point, deciding to ask before I yanked on it, lest it do something "interesting." I'd do a search, but I'm hanged if I can devise any sensible search terms.

I know the 12-v battery is back there, all the way aft, but the manual depicts jump-starting as an under-the-hood exercise.

 

I guess that would have been something "interesting"! Thanks.