This is normal for the Lumi Pro light unit — the circuit slowly self-discharges while it's idle, so the battery will drain over time even when the light is switched off. It isn't a fault.
The best approach is simply to charge your Lumi Pro up before each use, and top it up if it's been sitting unused for a while. If it drains unusually fast even right after a full charge, get in touch and we'll help troubleshoot.