The gun belonged to a sheriff's deputy who has since been put on leave, according to KTVU.
Jon Segarra said his 9-year-old son Josh and three other children found the gun in a bedroom drawer after they arrived at the home.
"That was just in a drawer in that room," said Segarra. "It wasn't hard to find. It wasn't hidden. It wasn't stashed under a bed or anything. It was in a drawer right there."
The family found the deputy's badge. Airbnb gave the family a full refund.