A forensic investigator arrives at a small apartment where two men have been found dead, officially from carbon monoxide poisoning caused by a faulty gas heater. The investigator notes four observations: (1) the gas heater in the living room is still running; (2) a backgammon set is laid out on the table with pieces in mid-game position; (3) no fresh fingerprints from either victim are found anywhere in the apartment — not on the backgammon pieces, not on door handles, not on drinking glasses; (4) the apartment's windows are sealed shut. The investigator concludes the scene may have been staged. Which single observation most directly supports that conclusion, and why?