Same 3l size pots i used in all my setups/grows (for around 200 plants now) and this is the first time i have seen this kind of spottings...
It started of not in the top or at the bottom, but right smack in the middle of the plant.
but only on its larger leafs, after the spots started to show the leafs slowly dryed out, they keept the fresh green coulor but feelt stiffer than normal.