Is the path to his bedroom from the garage/front door all hard surfaces? If it is, may want to take a slightly damp towel and wipe random spots along that path and see what you get up, and if you do get something does it possibly match the bedroom soiling. Have also done the same as Mr Ferris before. If attempted, make sure windows are open and you are getting lots of fresh air. May want to test a very small area first before going forward with it. Also can try putting a spray head to your ink remover. Had to do this recently as a landlord had 50-100 mystery ink spots throughout 2 rooms. Sprayed, extracted and buffed. Went through almost 2 bottles of ink remover (one was almost empty and dated 7/2003, just a reminder that if you don’t use something, still make sure you have ample supply because you never know when you may need a lot of it).