Diz, what BS is referring to is known as a "tell-tale"
Surveyors use them to detect movement. If the glass breaks after only a short time, then clearly movement is taking place. Further movement can be seen as the break in the glass widens.
Having said that, though, if it moves that quickly......... then it may be time to run!
Can you describe the location a little more? Does the beam span an opening through which you walk onto the balcony, or under it?
Does the beam run parallel to the balcony? I can't get the layout into my head.