I am seeing dark spots developing on the leaves of some of my hot pepper plants suddenly and seemingly all at the same time. These plants are being grown indoors under lights and are not necessarily near each other - in different locations. The affected part of the leaf has turned dark, thin, and feels kind of moist and "lifeless." Doesn't look like bacterial spot, and I've never seen this phenomenon before. Only thing I have changed recently in my grow area is the location of the fan. Could this be related to air circulation somehow, or more likely something else?

Thanks for your insights.