1. Magnet fell off
Yep. Fell off. Inside crankcase. I'd had the drain plug on a track/race bike for ~1.5 years, all seemed fine, and at my last oil change I noticed the magnet was "missing" from the end of the bolt...
The magnet was loose inside the bottom of my oil sump, I had to fish it out with a separate magnet on a flexy-stick.
Reviews elsewhere also note this problem. Steer clear.