Tell the truth, no matter what it is. If you have no idea where the money went, tell them that. Explain that given how long ago this occurred, your mother’s health at the time, the small amount transferred (if any), and your mother’s other resources at the time (if she had any), she certainly did not make a gift in contemplation of qualifying for Medicaid. You can just make the best argument you can and let the chips fall where they may.
For more on Medicaid's asset transfer rules, click here.