Diagnosis
This is almost always caused by water or moisture trapped inside the button area or electronics, creating a short circuit that "presses" the button on its own.
How to Fix It
You need to dry it out completely. 1) Take the brush head off. 2) Shake the handle vigorously, button-side down, to force any water droplets out. 3) Get a hairdryer, set it to COOL or LOW heat, and blow air directly into the seam around the power button and the charging port for 2-3 minutes. 4) Place the handle in a dry, warm spot (like on top of a fridge or router) for 24-48 hours. Do NOT put it on the charger during this time.
If That Didn't Work
If after a thorough dry-out it still ghosts on, the internal switch might be faulty. As a last resort, you can try to clean around the button with a toothpick and a tiny drop of isopropyl alcohol (90%), then dry again. If it persists, the unit is likely defective and needs replacement.
Heads Up
This is a sign you might be getting the button area too wet. When rinsing, avoid directing water straight at the button. Rinse the brush head separately, or just wipe the handle with a damp cloth.