Hey,,, sorry to here about the bug problem.
I have been fighting bugs pretty much through my whole growing life, and most of the time they are mites, or thrips, once fungus gnats. Its not that im not clean or keep it all tidy, its just where i live and grow is abit of a bug haven... Anyhow onto your question.
Well from my exp with spidermites when you spray you always spray the soil so it at least soaks in about 10mm at least. I have seen mites in the soil about 3 mm deep. I used a small microscope with the soil samples to view them at about 40x magnification. I dont belive they live or stay in the soil for long as they mainly feed on juices from the plant, but say you sprayed with water they will be washed off into the soil and try to make there way back up the plant. Using somthing sticky around the bottom of the stem will stop them going up the plant back to the leaves. Also as far as im aware they dont lay eggs in the soil, but they certainly do travel accross the soil to get too the plant. So always spay lips of pots and sides and i normally spray the whole room too.
Sorry if im ranting now,, take it easy and no mercy to the mites.....
ps. Iknow your in hydro but im sure they dont feed on roots, mites that is,,also it maybe could be the root aphid babies, as they look like they have eight legs, where were the legs postioned on the body will help id the mites from other pests,,,