OK...it doesn't count when some kid leaves his can on a ledge in the parks somewhere! ; I'm going to start out at DHS - because that's what that orange/red thing is making me think of...something in the vicinity of Pixar Place maybe?
Judging by the colors I'm guessing somewhere on the ride, not in the preshow area. I'm guessing that no one thought that someone would come along with a 21mp camera and be able to see that detail in the background on such an attraction. Kind of like in 2000, when some crew member left a bottle of Powerade on the stage for Fantasmic! (look to the right of Mickey, on one of the crates)