Unclaimed items should be deleted if they are left unclaimed for three weeks. Unclaimed items are set to being "NPC owned" and can be claimed by anyone that comes across them.
I believe Syn mentioned that they would be deleted if left in a random terrain square for three weeks without being claimed. It was in the Sim News Comments but I am not sure if the deletion aspect works or not because I have never tested it myself.
They are only deleted if they are unclaimed and out in the open for three weeks. If they are inside of another container they are never deleted even if unclaimed.
The deletion does not seem to be working currently; it broke around the same time monthly jobs broke about a year ago or so. Assume that it will be fixed suddenly, though, to avoid losing items you don't want to lose.