The idea was that Batman was supposed to arrive in time to save Jason's life; Jason was never supposed to die. So in this world, he died, and when Superbrat Prime punched time, he came back to life.
I'm not sure what kind of sensor would do that, but I personally just wave it with "Batman designed those sensors". Maybe they take into account pressure, but only when it's applied from outside, or maybe they'd sense if the coffin was removed, or maybe if the lid was lifted, or maybe they simply didn't register the breach because it was from the inside.