I believe it could be stolen. Maybe ask a police or staff member in game to check the chest logs. They will also help you what you need to do when it is stolen indeed.
You did lock the chest right? If you didnt people can go in the chests to steal. Always lock your chest by rightclicking a sign on the chest without shifting!
It was probably stolen, you can ask a staff member or police officer to check the chest logs and post them into a court case if you wish to sue the player who stole the things.