Sirius knows by now that it's likely Peter has seen the bruises. He thought he'd seen him catch sight of them this morning. It had been part of the reason he'd fled the Tower shortly afterwards, leaving his discard clothing behind.

He's not expecting any of them to be back when he enters the Tower. That's why he's halfway to taking his shirt off again when he walks in and is greeted by Peter.

Yanking his shirt back down over the rest of the bruises is instant.

"--Hey, Pete. What're you doing here?"