Hello and Welcome! Thank you for allowing me to be of service to you. Please note: (1) this is general information only, not legal advice; and (2) I will provide you with honest information and not necessarily to tell you might be hoping to hear.
I am sorry to hear about your situation. A place to start is to contact the financial institution in which the money market account was established. It is possible that this asset was not part of the trust and stood alone as a money market account with a payable on death beneficiary designation. If this is the case the financial institution would be able to assist with the transfer if indeed you are the beneficiary.
It would not hurt to have a lawyer review the accounting and trust agreement to determine it for accuracy and to outline your rights as a beneficiary of the trust. While the accounting may not have listed the money market account the trustee may still be aware of the account or information regarding the account.
Unfortunately, it is possible that your grandparents no longer held the account or it no longer had funds at the time of their death.
I apologize that this was probably not the answer you were hoping to receive. However, it would be unfair to you and unprofessional of me were I to provide you with anything less than truthful and honest information.
All my best & encouragement.
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All states have intricacies in their laws and any information given is simply information only and specifically is not intended to be, nor does it constitute, legal advice. This communication does not establish an attorney-client relationship with you.