missing document numbers

Back in August, our Accounts Payable department using transaction FB60 posted a vendor invoice. A document number was provided and it was handwritten on the invoice as proof.

however, there is no record of that doc number in SAP. It's not searchable in FB03, nor could I find it in any table (BSEG, BKPF, BSIK, etc).

Where did this doc # go and how can I find it?

Thanks

Josh