I am not stupid. I have seen this happening many times. Wiping themselves while I am getting ready to leave as the next client is only a few minutes...
Ah, after you? That's not proof. They might not shower right away, just wipe the icky part, text, snack, relax, and shower right before the next guy....
Big assumption to make that they don't wash once you leave. As for them using a wipe afterwards, they use lube, I'm sure it's not comfortable walking...