After a couple of minutes the natural vaginal lubricant that seeped out onto the customers pants will dry and get crusty thus leaving a "snail track" down the patron's pants.
DoocheBag 2: What for?
DoocheBag 1: I got a lap dance last night and that horny skank left snail tracks on my best pants.
DoocheBag 2: sweet! huh huh, huh huh
Esben: I saw a magnificent porn movie just before you came over, I didn't get to whipe my dick that well, so now my underpants are loaded with snail tracks!