Good question. Where would mites come from too?
In my dart frog tank various plants have popped up from out of no where. As best I can figure the seeds of these plants were probably in amongst the roots of the bromeliads that I planted in there. So maybe this isn't that big of a mystery.
Now the big mystery, where did the plants that are now sprouting up in the baby red-eye tank come from? There are no real plants in this tank, so there can't be any stow-aways in this situation. The substrate is ground coconut. In theory there should be no plants growing in it. Why? Because I treat all new substrate with boiling water. So if the seeds of any plant were in there, I would think that they would be killed by the excessive heat.
Another mystery, about six months ago I found a ton of mites in one of my scorpion enclosures. Where did they come from? They were crawling over everything, including the scorpion itself. I did notice that the lion's share was in the substrate, again ground coconut. They didn't seem to be harming her in any way, but I cleaned the cage out just to be safe. I took a damp Q-tip and picked the mites off of her. That was real fun, as she is quite ill-tempered and big. She's not even remotely friendly. I had to repeat this process twice, before I finally rid myself (and her) of these little arthropods.
So in the end, where do all these things come from? How come every spring before we treat and open the pool in the backyard I find clouds of daphnia in it? Not that I am complaining, they make the perfect newt/salamander larva food. In the end. life can be mysterious.