Wicketanswered grow question 10 months ago I had this too on multiple leaves and thought it would be some kind of pest or so, after hours of searching on all leaves with magnifier glasses and so on, i just couldn't find anything.
my guess it's coming from wrong balanced nutrients, root problems, or wrong ph/ec.
after knowing it's no pest stuff and it only happened on a few leaves i just ignored it and so far all good. if this happens in a larger scale i would start to worry.