At 3 minutes, 35 seconds, small guide-light burns out. The Actor wonders if some unseen thing has "probed a hole through the frosted glass," to let in the infectious air. But if a bulb, which has a vacuum, gets a hole, it will implode, not just burn out.
The theatre is supposed to be totally deserted, except for the Actor. But at 1 minute, 48 seconds, in the reflection of the glass on a machine, you can clearly see a crewman walking along.