Well if they are solitary and fast, and you are not seeing them on the leaves at all, then they don't really sound like parasitic pest mites to me. Maybe the mites are feeding on something else that is infesting the soil. I'm no expert on mites though; I'm just guessing based on the behavior of mites I have observed on my plants. If you have a good magnifier, like a jeweler's loupe, try to get a good look at one. The predatory mites often have forward-facing mouth parts, while the pest mites usually have more downward facing mouth parts. Also, maybe try to figure out what they are feeding on. Then you will know if they are predators or parasites.