The lights flicking off should give some idea of the trouble to come, but if not, the flitting shadow and sound of smashing glass would be a fair alarm bell.
Each Legend receives a mighty blow to the head. Maybe those chairs were for more than observing, after all.
When they awaken in their beds, it's with the mother of all headaches and a few cuts from the glass, but hey, they're free, and luckily, free of any major gashes.