Looking for some advice here.

In October 2013 Mrs PV_ and I each received a 1016 type voucher by automated email for a delay incurred on a UA flight -- totaling $300.

Today when trying to use the vouchers I received errors online, so I called web support and learned they had been consumed, but neither of us have used them and noone else has had access to the PIN codes since they were issued.

Web support and 1K line are useless, telling me that they can see they've been spent but cannot tell me when, for what PNRs, or by whom.

Who can I call to find out when/where these were consumed?