Walt wasn't suspicious.  He'd noticed her scratches once before and believed her when she said it had been an accident.  He'd have believed her now, if she'd offered up even a semi plausible explanation.  An attack by a bird in COMC.  A sojourn into the Forbidden Forest (although why she wouldn't ask the boys along he didn't know)  He wasn't looking for anything unsual, but if there was one thing that would make him suspicious, it was Brigitte trying to put him off.  She'd been trying for the last fifteen minutes to get those sleeves up.  Now she was pulling them down?  Not likely.

“Right. I am perfectly capable of doing this with sleeves."

"Fine, but what happened to your arms?"

“It’s not your concern,” she added

"Yeah? like hell.  Accio book."  He summoned the book she was looking at.  "Class is over.  How about we try this again and you tell me what's going on?"