Strange pauses for a moment, thinking. The way Childermass is asking him if there's something he needed is making him second guess why he's here to begin with and if he does need anything. After all, Strange knows that his memory isn't entirely the best and he's self-aware enough to realize that his concentration is just as terrible.

"I don't think there is." No, this is just Strange bothering Childermass for the sake of bothering Childermass. He didn't even know the man was out here until he came across him and the dog, practicing their act. Still, Childermass seems so sure of himself that Strange can't help but ask, "Should there be something I needed in the first place?"