"I'm a Prefect," Draco pointed out, pulling out the lapel of his robes to show his badge - not that he was on rounds, tonight, but he hopefully assumed that Tonks wouldn't know that.
And was she asking about his makeup? His sneer tightened a little angrily. "And were you lost Professor? Or simply trying to try a new morphing trick in the boy's loo? Either way, I'll gladly be on my way to finish rounds." And rounds already should have been over, but he hoped she wouldn't know that, either.
He didn't want to leave the loo, though, now that he'd remembered ... well, something. He wanted to have a chance to look for whatever it had been.