Drawing a peen on someone's face is always the go to graffiti.
NYPD guys have a "memo book" that is supposed to be signed by a Supervisor at least once a tour.
If you leave it unattended, very good chance someone will get ahold of it and draw a huge peenish on the back of the page opposite the one you are currently using to document stuff. WHat usually happens next is this:
Sarge: Hey. Whats happening. Let me give you a scratch (signature in the memo book).
Cawp: Sure, there ya go. (hands over book)
Sarge: (opens book) w...t......f.........
Sarge: (stares at cop)
Cawp: What. whu
Sarge: Dont leave your book unattended, ***thead. You've been donged. And its a gooood one.