You've done the right thing in requesting a copy. If one was filed, you will need to know its expiration date. If you cannot establish that a DEP was ever filed, you must otherwise determine a date on which the bank stopped filing CTRs.
In the interim, I suggest you conduct a documented review of the activity in the account and, if justified, file a DEP now. That filing will be retroactive for CTRs in the last 30 days and will at least "stop any bleeding" until you find out whether the form was filed previously.
If it's determined that the form was filed and has not yet expired, then you have filed two DEP's on the customer that overlap in their time frames. That's not a great sin and you will have an explanation.
If there is no documentation in support of the form's ever being filed, you will need to determine the date when you stopped filing (probably by manually looking through the CTRs that you did file). If the form was filed, but expired in March of this year or a prior year, you are looking at missing CTRs since that date. In either of these two scenarios, you will need to seek a
backfiling determination.