What does it mean when an old adult experiences sundowning, but has no other signs of alzheimer's?

It depends: Sundowning is a nonspecific term for a constellation of behavior changes that occur late afternoon or early evening. People also use the term for nighttime behaviors as well. This is not a symptom used to diagnose any illness.

See below: Sundowning is not a normal feature of aging, and can be associated with medication rxtns, sleep disorders, nutritional deficiencies, as well as associations with alzheimer's and other dementias. Need to get this assessed and handled, as could worsen fairly rapidly.