The plant this leaf came from is one of my larger plants. It is a little too dark, but the growth is ok. This was the only leaf like this so early days of something.
supper silver haze (not ideal for indoor growth, but the only photo period seeds I had at the time.)
Fox Farms ocean soil
top dressed 3 days before switching to flower. Top dressings was a mix of craft blend and worm castings
i think the picture will say it all to someone who has seen this, but let me know if you need more.
Leaf was found while trimMing off the lower leafs - first week of flower just started cleaning up lower part of plants.

Not sure why the post claims the pics were not uploaded.

Yeah my ph pen just went nuts on me started reading stupid low and then took much much longer to calibrate.
The dots close to even and following the leaf veins - maybe the plant was just pulling nutrients from it but also could be a virus? The plant looks fine. I also noticed that the other leaf stems are not purple, but this leaf was.
Thanks for the comment.
Rafe
Hi, it's hard to tell from looking at one leaf, can you get us a picture of the whole plant?
But if it is just one lower leaf I wouldn't worry about it too much unless you see it spreading.
I would double check the PH of what you are feeding and maybe add in some CalMag.