Formality wash

A very quick hand wash (generally not involving soap) by a person who has just urinated in a public restroom, expressly to let others in the restroom see they haven't left without washing, when otherwise - had there been no one else in the restroom - they wouldn't bother

Restroom patron 1: "That guy barely got his hands wet at the sink, then walked out!"

Restroom patron 2: "Yeah, that's a formality wash"