This story was updated at 4:13 p.m. to include comments from a San Diego County sheriff's deputy.
UPDATE 11:45 a.m. Oct. 5:
Derek Thompson was back at home with his family Friday, according to San Diego County sheriff's Detective Thomas Vrabel.
"Let's just say he was gallivanting around our two counties with a friend, in a vehicle," the detective -- based out of Fallbrook -- told Patch.
"Both of their cell phones allegedly discharged, thus no contact could be made. Once they returned to Fallbrook another friend, knowing the missing person status, brought him to our station for a discussion," the detective said.
Thompson was examined and his name removed from the national missing persons data base, Vrabel said.
"He seemed like a real nice young man and was quite apologetic.
"His ... mother met him at the station for the reunification," the detective told Patch.
ORIGINAL POST Oct. 4:
The public's help is sought in locating an at-risk missing Fallbrook man.
Derek Sean Thompson, 20, was last seen by his mother at about 7 a.m. Sept. 28, according to a flier being routed by the San Diego Sheriff's Department Fallbrook station.
It was reported that he may have be seen Tuesday in Murrieta, however that has not been corroborated, according to sheriff's Detective Steve Ashkar.
Thompson has not been in contact with his family, nor has he ever gone this long without contact, the flier stated.
Thompson reportedly has a severe learning disability with the mental capacity of a 13-year-old, having just graduated high school at the age of 20. He is not on medication.
He may possibly be with companion Elmer Leonzo, 22, according to Ashkar.
Leonzo is who Thompson was reportedly with during the last credible sighting at about 9 p.m. Monday at 1455 Alturas, Apt. 1 in Fallbrook.
Leonzo drives a black two-door 1996 Nissan Sentra with California license plate number 5XRW054.
Anyone with information regarding Thompson's whereabouts was asked to contact Detective Ashkar at 760-451-3111.