Not much of a secret. He'd gladly tell anyone it was just always best to keep ones cards in hand and never all out in play. Though Evie seemed to have a good idea of how to play that game.
"Don't mention it. My name is Jefferson, for future reference. When I've got something for you I'll send a message." It was easier to assume someone was coming for tea. Not to mention, the Hatter didn't rightfully venture out of a usual schedule. Many ways to be suspicious.