Tell the truth - you think she is either genuinely gone or is hibernating or lost. Explain she is a pet snake, and docile, though that may not reassure a person who is phobic. Any chance of getting a "pest removal" expert in to find her, explaining that she is a pet, and not to be harmed, just found? How long has Merle been missing? How likely is it that she's hibernating, given that it's summer, and warm out?