Polaris Vac and Magnets inside

I have a Polaris (kreepy krauly) style vac... one of those types that randomly vibrates around the pool floor. I noticed that it has some magnetic components on the opening that sucks up the debris. Of great curiosity is the fact that over time these magnets pick up a fine magnetic powder which I wipe off. I can not imagine the source of the magnetic filings, although as I write this I was wondering if the iron is coming from my Frog system?

I only use liquid Cl and the Frog system, and the only thing I've ever put in my pools is baking soda, polyquat and stablizer (when I first filled last year).

Any ideas on the source of magnetic powder would be appreciated.

Thanks

18x36 IG vinyl
24k gal
200 lb Starite sand (with a little DE ;) )
Starite NG heater