"There's usually a key," he starts, "Or something must be done before the seal is undone."

He says this like it's the most common thing in the world. Which, to be fair, was true of Hyrule and the Twilight Realm. But he's forgotten again that not all realms worked the same as the ones he's visited. Link just assumes that Korra would figure out that his reason for inspecting everyone's(?) houses was for the purpose of finding a way through the web. Actually, now that he thinks about it-

"... Have you passed through the web downstairs?"

Because he hasn't yet.