The letter probably came from the lender and/or loan servicer - try them first and see if they sent something out.
Lenders will typically require an annual Occupancy Certificate. One of the conditions of a reverse mortgage is that the borrowers occupy the property as their primary residence. While a second notice may be sent to your parents, contacting the lender might be the best bet.
Call the company that's servicing your parents Reverse Mortgage, and let them know. The toll free phone number is listed on the first page of their monthly statement.