Bava Batra 70 - Returning a Deposit

It has been established that one who accepts a deposit in the presence of witnesses is not required to return it also in the presence of witnesses .

What is the rule if one records making his deposit in a document, and the custodian tells him, "I have already returned it to you"? Do we say that if the custodian wished to lie, he could have said that the deposit suffered an unavoidable mishap, in which case he would have been believed? Therefore, we believe him on the strength of "why would he lie?" argument?

Or can the depositor ask, "Then why do I have this document in my hands?" - so "why would he lie?" is not applicable?

Perhaps, the custodian didn't bother retrieving the document, because he knew that he could claim unavoidable mishap and would be believed. Therefore, he is believed now.