Tenant's Mail Stolen From US Post Office Provided Locked Mailbox

[Radley Estrada]

Hello fellow BP'ers. I own a 4-Plex apartment. There are 3 neighboring 4-plexes, and we all share a US Post Office provided cluster mailbox that is located outdoors on the sidewalk. A tenant called me and complained that a package was delivered to his mailbox, but, when he went to the mailbox - there was no package and the door of his mailbox was open. Only he and I have keys to access his mailbox. Anyone else have a similar tenant experience? Who is liable? What should be done next? Should I change the locks? Thank you guys!