If you actually watch Truman in the movie, you'll notice he had already started building his escape tunnel. The entire movie isn't him learning everything is fake, it's him trying to stall everyone while he figures out how to get out. He's testing the boundaries.
He escapes through the tunnel they find. That's how he gets to the dock to steal the boat. It doesn't show him in the tunnel. He's just gone for awhile evading everyone, and eventually they find the tunnel in the basement.
Are you sure you aren't thinking of Shawshank Redemption?
I'm just kidding. I haven't observed what you described, but I'm sure it applies. Apologies if it sounded dismissive. Next time I watch the movie I will watch for what you mentioned and thank you for drawing my attention to it if I notice it!