DIAGNOSIS That's weird, but it's a known glitch with some units. The light is supposed to only come on when the trigger is pressed. It staying on indicates a fault in the switch or control board, not the LED itself.
FIX The first thing to try is a hard reset. Take the battery off and leave it off for at least 2 hours. This sometimes allows any residual charge in the tool's capacitors to drain, resetting the electronics. Put the battery back on and see if the light behaves.
IF THAT DIDN'T WORK If it persists, it's an internal electrical fault. The switch module or the small circuit board that controls it needs replacement. This isn't a user fix unless you're very skilled with electronics repair and can source the exact part.
HEADS UP Yes, it will very slowly drain a battery if left attached, as the light is powered. Always physically separate the battery from the tool when storing it if you have this issue, until you get it repaired.