The 21-year-old was last seen in Dunmore Crescent in Cookstown, County Tyrone on 7 October.
On Wednesday, PSNI Supt Michael O’Loan said it was “completely out of character” for Mr Johnston not to have been in contact with his family.
The remains of Mr Johnston, who was from Coleraine, was by a community search and rescue team in the Maghera area.
He was being treated as a high-risk missing person.
A 31-year-old woman and a 33-year-old man arrested following his disappearance were released on bail on Tuesday night to allow for further enquiries.Mr Johnson was last seen at 5.24 pm on Saturday, October 7.
CCTV images showed him at a supermarket on the Orritor Road in Cookstown around half an hour earlier.