Is that the only screw loosing up? It's hard to say at this point in my opinion but I would definately be keeping a close eye on BOTH sides of that rear cap!
Maybe Porthole can chime in with an opinion as to how his rear cap came loose and what his opinion would be.
Screws backing out on DRVs seems to be a wide spread issue, we had SEVERAL back out on our '15 RSSA! Along the sides rear of the wheel well on the off door side, along the rear cap on both sides and underneath by the pinbox. I had a couple of screws land in the pick up bed.
Keep an eye out, good luck!!
Dan