Usually is found after one has defecated in the restroom and has left a little extra residue on the seat. Tends to look like a little yellow/brown strip and most commonly is found on the back of the toilet seat, where one's butt crack sits.
Travis: "Hey, didn't Derek drop a deuce earlier?"
Mark: "Yeah, I believe so, why?"
Travis: "That douchebag left a buttertrail on the seat!"
Mark: "Yeah, he tends to do that. He never wipes it off either."
Travis: At least he didn't shit in my underwear and leave it under my bed like Mike did."
Mark: That's true. Mike is savage as fuck!