Officers investigated a robbery at a community hall in Wodonga, in south-east Australia, at the weekend.
Instead of fingerprints, police found the offenders had left a bare bottom mark on a glass door.
Sergeant Shane Martin, from Victoria Police, told The Age newspaper he had never seen anything like it.
"They're having a joke, or they're actually trying to lean up against the door to break in, but I don't know why they'd do it pantsless," he said.
The intruders set off a fire extinguisher before stealing a television.
Police are looking for three people spotted in a black car near the scene.
Wodonga is a town about 300km (180 miles) north-east of Melbourne.
Police dusting for crime scene fingerprints rarely encounter so much cheek.