sleeping turd
A turd which is not noticed when encountered, but presents evidence of its existence at a later time.
P1> Damn, What the hell is that stink?
P2> I think it's my backpack. I must have set it down on a sleeping turd when we were at the park.
P2> I think it's my backpack. I must have set it down on a sleeping turd when we were at the park.