Anyone know home and contents insurance?

The homeowners policy would not be “voided” by an open house event. However, you would have to establish that your loss was the result of theft. Just the fact that something was gone is not enough. Perhaps the item was misplaced. You would have had to have some suspicious circumstances because mysterious disappearance is not covered by your typical policy.

Even if you can reasonably establish that your loss was because of theft your policy has a deductible so if the loss was small it would fall below the deductible.

If the item was jewelry the typical homeowners policy limits the theft loss to $1,000. unless you had the jewelry scheduled on a floater in which case the jewelry would be covered in full and even for mysterious disappearance.

Obviously the answer has to assume a typical policy so call your agent to verify the details of your policy.