Visible crew/equipment: Near the beginning when the "cavemen" are looking around after hearing the noise from the alien, you can clearly see what looks like a sneaker print in the ice, in 35,000 BC. (00:03:00)
Visible crew/equipment: Near the entrance of the building, when the fire brigade arrives, a guy holding a boom microphone is visible. (00:19:25)
Chosen answer: If they had put an "Out of Order" sign, or left it unilluminated, someone in charge might try to have it fixed or replaced, or could have checked to see if it's plugged in, etc. By leaving it illuminated, it doesn't look out of place and simply not taking money wouldn't arouse too much suspicion. In my own life, when I've come across a seemingly working vending machine that won't take my money or dispense drinks (but gives my money back), I just find another one instead of calling someone or reporting it. But yes, it is very convenient to the plot that Mulder just happens to try and use this particular machine.