Not every leaf will look perfect during a grow... It could be anything really? Symptomatically Rust spots are usually Calcium Issue(but, too much/too little will often give the same leaf signal)... if its a lower leaf it could've been something that happened weeks ago? and chasing something that happened weeks ago is chasing your tail. Always look at new growth, if new growth is healthy and the node or 2 down from the new growth, your OK, if it becomes wide-spread you have a problem, I would be more suspect low calcium just because its pre-charged soil, and your plants look about 6 weeks old. - You should add Cal/Mag every other watering, especially going into flower (soon). You will eventually move away from "pre-charged box store soils", because you will never have control over your process, You never really know what's going on in the soil media. Experienced growers will never sweat a leaf or 2, especially lower on the canopy, get some cal/mag, monitor your Plants daily. that soil will run out of nutrients soon.? what is your nutrient plan? Overall your plants look in good health, Good 1st grow so far! Right On!