For me, it's gazing deeply into the eyes of the guy at the urinal next to me before releasing an earsplitting, raucous, filthy putrid fart. Then I take a couple of exaggerated whiffs, give him a light punch to the shoulder and stroll out without washing my hands.
If someone's in the toilet stall, I sometimes bang on the door unexpectedly on the way out.
If someone's in the toilet stall, I sometimes bang on the door unexpectedly on the way out.