As the others said: I would inform Apple, but it is not your job to do anything more. If the underpaid guy in the call center could not care less and does not really feel like looking up if this iMac is missing somewhere, I think you have done your duty.

Put it aside for some weeks (for example till Christmas). If they do not call back and arrange a pick up, simply keep it. Write down when you called them (you potentially do not have the name of the person you talked to?). That should be it.

If they streamlined their processes so much that nobody has the time/gets paid to solve unusual cases like this one, you simply have the benefit for once...