Not thinking of which you can't clearly identify, I am thinking actual identifable sounds, like in "This Town Ain't Big Enough for Both of Us" by Sparks which uses gunshot sound effects. There are probably a million tracks with gunshots and police sirens.

Or "A Survey" by Tortoise from Millions Now Living... where the band inexpicably back up the entire track with the sound of crickets.

Any particularly imaginative/inventive examples which really add to a track?