By the looks of that leaf, it could very well be spider mites. Have you seen them? I'd scope underneath some of the leaves in affected areas for a positive ID to confirm what you're battling. The worst thing with spider mites is that they often multiply unseen, then it's like overnight that your grow becomes infested with webbing everywhere. You mentioned no webs, so either you're in the early stages or they aren't spider mites.
The soap will hurt their ability to breath properly, and although it likely won't kill them, it will at least help keep their numbers under control. Forceful spraying of all the undersides of the leaves will knock them off too as well as raise the humidity (which they don't like). Keep lots of air blowing around in the tent as well, and try keep your temps under 30 C. Spider mites thrive in still, hot dry air. If the species of spider mite wasn't introduced by another cannabis plant and instead tracked indoors from outside, it's likely something that isn't adapted to cannabis and might be easier to manage. Sometimes even by fixing environmental parameters you can manage to control them if that's the case.
If they start to get worse, you may have to take more drastic action such as alcohol (ISO) and water mixture spray. Keep in mind this will hurt the leaves to some extent, so only do that as a last resort. Got any ladybugs available? There are other predatory mites you can buy for them as well, but a friend of mine who tried it found they weren't very effective. Anything is worth a try though when it comes to predators. Just find out what's available or what you can catch in your area.