First time mites - came from dispensary clones
Second time mites - came from seed package I suspect.
Mites are actually pretty ubiquitious, they can just show up, when they sniff out their favorite plant.
If you don't have them now, and you don't take in any outside clones or grow stuff, you are probably safe till the winter ends.
Personally I never assume I don't have them, and though I see no mites nor signs of their damage right now, I'm releasing some longpipe mite predators tonight as well as some sternimala feltiae for gnats (I see one or two of those though) tonight.
Remembering how tiny they are it's, IMO, pretty much impossible to totally keep every last egg/infant/adult from ever entering the grow room. At least that's true of the way I built my garden, and every home garden I've ever seen, it's not really hermetically sealed just pretty well sealed.
One of the things we (as mj growers) do - especially in the flower room, is keep it hot and dry all the time. That's the ideal environment for the bad kind of mites, and a poor environment for the good guys. Pushing up the humidity is tricky because of mold, but 10-20 degree downward temp swing absolutely helps a lot as a preventative measure. If I had an active infestation I wouldn't hesitate to go down even a little further (my daytime is 82) maybe even push down to 55F night. That does effect yield a little, but it brings out beautiful colors!
Other than that? Ice-water, showers, predators, cleanliness, removing any damaged tissue, and most importantly growing healthy plants with strong natural defense mechanisms is the key to organic mite control. Follow these steps and mites will never be a problem. Mites are by far, IMO, the best pest to have in your garden.
A 20% CO2 concentration for 7 hours will also take care of mites, easily achieved by bagging small plants and displacing the oxygen within the bag using a CO2 bottle. That's probably the quickest way, if you've already got the bottle. I do have an academic reference, if anyone doesn't believe me, but it's easy to try for yourself.
Predators: lady bugs, infant mantises, and a number of predatory mites, other general garden predators.