The combination
Cindy realizes that she obviously knows the code to open up the padlock. She had found the chest after following the coordinates that were hidden under a layer of paint. The paint was part of a painting that Paul had made, recreating their favorite picture from their wedding.
The combination to the padlock was of course… their anniversary! Cindy enters the numbers, with success. The lock opened up and Cindy took it off the chest. She could feel her heartbeat in her throat as she carefully took the lid off the metal chest…