I apologize in advance as I am not used to dealing with deposit side issues.

We have a customer who had their checkbook stolen. We determined that the checks were not signed by the account owner. Where do I find information on our responsibilities? I know this does not fall under Reg. E because the transactions were not electronic. Some of the transactions overdrafted the account.

Thanks

The info you want is under the Uniform Commercial Code (under you state's statutes)= Section 3 Part 4. 3-404 deals with forged checks.

The short answer your bank had until midnight after the day the item was presented for payment to find the forgery and return the check as such. After that deadline the bank is on the hook, not the customer.
The customer does have to meet some limits of examining the items in their staements (UCC 4-406) but it sounds like your situation is before a statement was even generated.