Did you resolve? Likely to be the Hall Effect or Reed Switch that is situated on the left side of the laptop on my 2012 Pro Retina. The magnet on mine is in the lid. If you have had the lid apart then that will be where the magnet goes. Try placing a magnet around the keyboard perimeter and see if it goes to sleep with it in some place (others have found that putting a phone case with a magnet by the side of the laptop has caused theirs to go into sleep mode). If it does go to sleep then your mysterious magnet will be the problem. If not, then the sensor itself may be at fault