I think if it was RSM you would have had some come onto your hand while handling the plants as they do this for warmth. I have only had it on my Show leeks and i have never seen marks like that on them. They tend to do for the stems and extract the sap. Lastly when you have the mite there is no mistaking as they are hundreds of them, the size of a grain of sand.
I also think it is to early in the year for them