The dealer's asking to check their records is not unreasonable, but as far as the customer is concerned it is an internal issue for the dealer, and simply means that the dealer requests some more time to look into the matter, which should reasonably be granted.
But this is should not be taken as an agreement that the dealer's internal records will somehow decide the outcome of this dispute. It should not be accepted as final 'judgement' as to whether the disc was in the drive or not - the customer is convinced it was, and that's all there is to it.
From the customer's point of view, if the dealer is asking for some more time to check their records, talk to the technicians, look for the disc, etc, this is absolutely fine (as long as it does not take too long, of course).
So it is reasonable to allow the dealer time to look into this properly, and potentially find the missing disc, or decide on providing a replacement, rather than insisting that a brand new disc is immediately supplied.
But the outcome of the dealer's internal enquiries should not make a difference to the customer's request which is simply to have back his disc (or a replacement), regardless of the dealers' 'findings'.