Barty could be a lot of things. But careful was not one of them. He and his other intelligent friends had decided to sneak out at night, not knowing that there were several death eaters roaming around Hogwarts thanks to the war. Being also really attentive to his surroundings, Barty didn't notice any of it and instead had the brilliant idea to separate himself from the group to do a prank on them. Slytherins weren’t known for their intelligence after all.