Theo flicked eyes over the chair in the matter of less than a second - normally he would have done a covert charm to check the chair, but he felt (maybe foolishly) that he could trust his tutor to not have charmed the chair.
He took a seat with a 'Thank you Sir' and let the box move out of his hands as his teacher summoned it over, taking the praise with a carefully blank face but secretly banking it under his most important memories of the few times he was praised; he'd keep that one for months to come.
"No Sir. I didn't recognise the owl and there wasn't a note." He watched Snape cast over it and watched curiously to see if anything happened.