When you put your laptop in a bin, the security person puts one of two identical tags (pieces of plastic with identical numbers on them) into the bin, on top of your computer and gives you the other one.
When you finish your patdown/metal detector etc screening and want to take your computer back, you give the security person at the other end of the line your tag, she looks around for the bin with the identical tag, and hands you the contents of the bin and takes your tag back.
Not foolproof/fraudproof, but it seems to work in practice.