For his part, Sirius thought he was being discreet about the whole thing. It isn't really until he's caught that he realizes that he might be losing his touch, or that there are people out there who are better than him at being sneaky. Either way it comes as a surprise to be confronted so suddenly, though not outright, which gives Sirius time to come up with some excuses.

"Something to do, yeah," he replies, innocently. He didn't say they were going to be good excuses. "I mean, they're kind of old." And a really terrible picture of him.