Dropped Item in hot water heater
Since you have a flue I assume it is either gas/propane or oil fired. Most water heaters have cover near the base to service the burner or the igniter. First shut off the fuel source.  If you remove the cover you should be able to look inside with the aid of a flashlight. Most burners are round and you may find that the cap has fallen into the center and may even be on the floor under the tank.