This will be a function that was set by the machine tool builder, and depends what the light is actually monitoring, and what the spindle off circuit does, is this a Fanuc spindle? or other make?
If you have a print out of the ladder, and go into the ladder screen and find the output that turns on the light, the logic string it is in should offer some clue as to what is turning it on.
Normally does the light come on normally when the spindle is stopped? or after an M5? or only when there is a fault or a manual stop?