-- you may have found a note from Dagny saying that Gunnar has something interesting he's using as a coaster. Good grief!
Nancy won't ask about it, though. The only way to get it is to wait for Gunnar to leave.
There are two times this might happen. If you go to the ship and Nancy says she sees Gunnar going out to sea, you can dodge back to the pub and he'll be gone. However, this doesn't always happen, or it may have already happened and you missed your opportunity.
The other time is later in the game, when Gunnar has an argument with Elisabet and gets thrown out. When that happens, you have as long as you want to go investigate his spot, because he won't be allowed back in until you fix things for him.
Either way, when Gunnar is gone, go to the spot in the bar where he usually sits and take the coaster. While you're here, stop and read the letters on the counter as well; they're between Elisabet and a friend.