(This typically occurs with audiobooks.)
If an item is returned and missing a part, do NOT discharge it. Check to see who it is checked out to (check item status) and use the barcode you find to look up contact information for the patron.
Contact the patron and alert them of the problem. Often they have just forgotten it in their car CD/cassette player. If you do not speak directly to the patron, do not disclose the item title. You can only say that an item that was returned is missing one of its parts. Put a note on the item and place it on the shelf in the circulation workroom.
If you do not feel comfortable contacting the patron, you can forward the item (with the necessary patron info) on to Chris, and he can handle it.
When the patron brings the missing part back in, replace it within the item where it belongs and discharge as usual.