Failure to give up possession at the time the bailment ends could result in an action for conversion or negligence.

Conversion

The unauthorized possession or control over someone else's property, converting it for one's own use.

Possession

The act of having control over an object or property, with or without legal ownership.

Bailment

The temporary placement of control over, or possession of personal property by one person, the bailor, into the hands of another, the bailee, for a specific purpose.