Might be from the inside? , looks like someone spilled their drink behind the booth and can’t be reached .
Can you tell if it is on the inside or outside surface? What have you tried so far? A simple quick compound found at the hardware store might work - “Amaze Clean & Green”; use with wet white 3M pad or extra fine bronze wool or 0000 fine steel wool. Be sure it is wet and test a small inconspicuous area first.