I recently realized while levelling an alt that her whole armor was in the red. the problem is, the little durability guy never showed up. I checked the movers and it was nothing to be seen.
The little guy was there in the default WoW interface and its mover was absent from other characters on which I had previously seen the bugger.
Enclosed is a screenshot taken, I believe, with no other addons than nUI running. I know I checked that particular scenario, I'm just not so sure if it's the one I saved.