I'm trying to see this from both sides. Looking at details, your son was right about being able to hear the fire alarm. On the other hand, looking at the big picture, the teacher was right, and the details are irrelevant.
At the time, she was probably very stressed out, especially if the same thing had happened the day before. She may have been feeling flustered. This might have affected her ability to quickly throw out a reason for why it can be dangerous to hide in the closet (I can think of a couple. What if he knocked something over while trying to get in/out/hide better, and hurt himself? What if he hurt someone coming out? What if...etc?).
So she threw out a reason that wasn't accurate. Doesn't matter. The big picture here is that she was right overall. She definitely wasn't trying to open a debate with your son.
So even if he was right about a little detail, it doesn't matter. Plus, he could be interpreted as meaning, "I can hear the alarm, so it's okay to hide in the closet," especially to a person who's already stressed out. She probably knows at this point that you can hear the fire alarm in there, but her main point wasn't "FIRE ALARM." It was "DANGEROUS." His correct response (IMO) would have been, "I can see how my actions could be dangerous; I could have opened the door into someone who didn't know I was there when I came out," or "I could have knocked something over and hurt myself."
I can see that in that situation, a person wouldn't be open to hearing an explanation, because it could come across as an excuse for behavior, even if it wasn't meant that way.
There's probably an important lesson here: when someone is justifiably angry at you, a good way to soothe the situation is to see the big picture, acknowledge that the other person is right, and not argue about minutiae.
