There are several (what I’ve viewed) as clues in the seclusion scene. The way Lebowski reacts when Dude says “that’s a bummer” sounds a lot like he’s dropped the act, almost as if he was caught off guard by the way Dude responded to the kidnapping and forgot to stay in character. There’s also the way Brandt kinda just stands there waiting for his cue to hand off the ransom note.

But that’s just, like, my opinion man