Done with his forgery, Florian considers whether to board now, and risk close inspection, or later, and risk all the hiding spots on the boat filling up. Florian takes a moment to wonder what will happen to all the art stolen by the Nazis. He knows someone will eventually go looking for the Amber Room is the “greatest treasure of all.” Florian knows Hitler will look for the swan first, but Florian is the only one with a map to the cellar where the Amber Room is hidden, and the only one with the key to access it.
Florian stole the amber swan as an act of revenge against Dr. Lange and against the Nazi Party as a whole. The swan represents his total rejection of their regime and doctrine. He took it because he knew it was the most precious item in the amber room.