It's not that he didn't think Arthur would notice; there were a lot of factors to the party that would tip him off. The fact that he went with another person. The fact that he'd been wandering and a little listless during. The fact that he hadn't come home with Arthur.

He'd expected this to maybe happen later, the discussion, but he should have known better.

"Nothing useful," is what he decides on for an answer, which is the truth, in his eyes, but he's well aware that Arthur will delve. At least now he'll know that he's delving against the current.