Towards the middle of Unsane a body is discovered in the woods by a jogger and their dog. The police is later led to David/George due to fingerprints found on the body.
IIRC at this point in the movie, the undercover reporter has been killed and his body has been found in the basement (plus the hand is white) and Sawyer's mother's fate is unknown - David has been to her room and Sawyer can't contact her.
When I saw this scene I thought they found her mother, but later Sawyer discovers her mother's corpse in David's trunk. To my knowledge there's no one else who has been killed or ends up missing.
When writing this, I came up with a potential identity for the corpse - George - the healthcare worker that David impersonates, but as it has been a few days, I'm not entirely sure.
So who is the corpse?