Whenever you brush it after cooking, you're creating tons of tiny metal particles, from the grates, from the brush... those particles floating around will land on and become stuck in the residual grease on the cooking grate bars. I'd bet those particles are all that has rusted on you. If the WSM had some standing water and was also exposed to heat, such as from being in the CA sun, it would effectively become a big humidy chamber, so the rust is not terribly surprising. I wish you well, but I think you'll be fine with just the steel wool.