Turns out I've got quite a fancy magnetic sump plug ("King" brand, looks like stainless steel with a magnetic insert) and it had a fine slurry on the tip when I recently changed the oil, but no big fragments.

I have to wonder if these plugs are such a great idea, though.

The wear particles on the tip were definately magnetised, since some could be removed by touching it with a paperclip, on which they adopted the characteristic "whiskery" appearance.

If these magnetised particles get dislodged from the plug by turbulence in the sump, they might tend to stick to other ferrous surfaces (eg crankshaft journals) , and probably wouldn't be removed by an oil drain.

This might cause accelerated wear, and might be why these plugs aren't (AFAIK) fitted as OEM.

I'd say magnetic "filtration" is more likely to be a good thing if its fitted somewhere it can be regularly checked and cleaned (to reduce particle magnetisation due to long residence) as on the dipstick, and/or where its upstream of a physical filter (which might retain magnetised particles), as with a spin-on canister jacket.

I left a magnetic pick-up tool down the dipstick hole when I didn't use the car for a few days, and it picked up a light coating of slurry too.

Since I'm not running in (in fact I'm closer to running out) I might retrofit a standard sump plug.