After Marley brains Harry and Marv with his snow shovel, he turns the two burglars over to Officer Devereux. Devereux doesn't really have much to say, but functions as an information tool, letting us know what Marv and Harry's fate will be.
He tells them:
OFFICER DEVEREUX: Hey, you know we've been looking for you two guys for a long time. You guys are always leaving the water running whenever you break in, now we know each and every house you guys have hit.
While Marv thought he was leaving a "calling card" he was actually providing a trail of evidence, totally incriminating himself and Harry. The message is clear: these criminals are idiots.