You just want the puzzle solved!
I think you are missing something important: the guards were not together in the factory, they re together there for the questioning, but each one of them was alone during his shift. It doesn't matter their position in the picture, that's irrelevant!
From this you realize that only one of them heard the explosion, the one that was working when it happened. And since Bernie is the only one speaking about it (and by looking at his face you know he is stressed) he is the only one that could have heard it.
But this is just the set up of the puzzle, it's common sense, from the time they tell you they work by shifts you should aready know this. If you didn't get this, it's less probable that you could figure the real logic of the puzzle.