There are several considerations to the question.
- After hours, no one is in the room - there is no need to evacuate anyone
- Burning plastic emits highly toxic smoke - just a few breaths of smoke can kill
- The room is fairly large, presumably one can go far enough in that smoke can get between the security guard and the exit
- You mention firefighting equipment, but you don't mention personal protective equipment, including breathing apparatus
- A life is far more important than stored cardboard and plastic
- Paying for a false alarm is far less costly than filling out the paperwork explaining an injury
In the past, I've talked to a fire marshal about this. He said he'd suspend any of his firefighters that went into a burning building without first putting on the personal protective equipment, including safe breathing apparatus. He did not want any of his firefighters to be added to the injury list.
My personal suggestion to the security guards (you may wish to put this in their emergency response procedure book) is that if the alarms sound off:
- Call the emergency response personnel right away. That starts the fire fighting process.
- Unlock the door to the room, but do not enter for any reason.
- Stand back and let the emergency response personnel do their job.