McComb police are looking for an Alzheimer's patient who has been missing since noon Sunday.   Ruby Lee Wells, 72, 1109 Moore's Ave., McComb, was last seen at her residence, a police spokesman said.   She is a black female, 5 feet, 10 inches tall, and has a normal weight of around 170 pounds. However, she has recently lost a lot of weight, police said.   Wells has a history of wandering off.   People with information about her whereabouts are asked to call the police department at 684-3023. All the above information has been entered in the NCIC computer