Any chance that door was purchased from a wrecking yard? They will often mark the parts with a bright colored paint pen to identify it as one from their yard for future returns, warranty, etc. But I would have expected it to be in a more inconspicuous place like on the bottom of the door.

Check for witness marks around the edges of the hinges and the hinge bolt heads to see if there's evidence of the door being replaced or re-adjusted.