The phrase "mysterious disappearance" means that someone has vanished without any clear or understandable reason and it is unknown where they have gone or what has happened to them. Full definition
Most policies have a set limit established for a loss caused by mysterious disappearance or theft. (armourinsurance.ca)
This could also fall under mysterious disappearance subject to the policy limit for jewelry and the deductible. (thebalance.com)
One example is mysterious disappearance coverage or all risk coverage which covers everything that is not «excluded» in the policy wording. (thebalance.com)