missing savings bonds

My mother recently passed at 90 years old, and, as her executor I’ve been reviewing all her financial accounts before distributing to the beneficiaries (my sibling and myself). Over the past 10 years I had been assisted her with finances, learning where everything was kept, the banks and other locations of her investments, etc. I maintained monthly bank statement reconciliations on her main checkbook. However, she had a Money Market account that she never allowed me to reconcile. In reviewing that account, everything seems OK. Except one thing. According to the bank statement, in December 2018 she bought $700 in savings bonds. That’s very odd as she hasn’t bought bonds since the 1960s. I asked the bank if they could track this purchase down, and at the very least provide us with the serial numbers, but after 10 days have not heard from them. I went to TreasuryDirect.gov, and used the Treasury Hunt feature, but it returned nothing. We have not located any bonds in her personal effects, yet. Any suggestions? Thank you!!