If Dean saw the flinch, he gave no sign. And really he'd expected it anyway. Slytherins weren't on friendly-smack status with muggleborns, after all. Nott would probably have to run and shower or something now. But the other bloke had been friendly - if you could call it that - so Dean did the same.
He threw Theo a quick grin and a wave and then started down the stairs, leaving Theo to leave whenever it suited him. He wasn't a Prefect, he wasn't going to try to tell anywhere where to go if they didn't want to.