DIAGNOSIS The light circuit is stuck 'on'. This is a known capacitor issue on some runs of this model. A capacitor in the circuit holds a charge and keeps feeding the LED even with no battery.
FIX It's a simple discharge trick. With the battery removed, pull and hold the trigger for a solid 30-60 seconds. You're draining the residual power from that capacitor. You might see the light dim and go out while you're doing this. Once it's out, you can release the trigger. When you put the battery back in, it should behave normally—light on only when the trigger is pressed.
IF THAT DIDN'T WORK If the light stays stubbornly on with the battery *in* but the trigger not pressed, the trigger switch itself might be faulty or have debris causing a partial short.
HEADS UP This isn't a sign the tool is broken, just a quirk. If the discharge trick works, the light might occasionally do it again in the future; just repeat the process. It doesn't affect driving performance.