Older theatre designs place the return air vent for the air-handling unit behind the screen. Think of how many cubic feet of air passes through the screen each day. That air may contain dust, dirt, pollution and smoke. The theatre screen acts as an imperfect air filter for these contaminates. Matters are made worse when the air filters are not changed in a timely manner.
Excess pollutants circulate through the auditorium.
Dirt clings to every surface. If the drapery, air vents, and masking are dirty, then imagine the condition of the screen. Add to that the static electric field created by the speakers. This field attracts dirt too. Like iron filings showing the magnetic field of a bar magnet, an outline of the speaker cone may appear on the screen due to this field.