No payee endorsement, no bank of first deposit endorsement, or no endorsement, period?
Regardless, I can't help you with the digging, but can point out that the bank that accepted it without a payee's endorsement guaranteed the missing endorsement. If you can trace it back to that bank, it's theirs to resolve; i.e. you can put your customer in touch with them and step out of it. Your bank was not required to return the item "endorsement missing."
Don't assume liability or offer assurances to your customer at this point...just start digging.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.