Just because you don't see them does not mean they aren't there.
I had this issue with tiny little flying bugs, although I didn't let it get to that extent. Just put some fly papers up in grow room and this cured it, also found there was a fair number of them but they were so tiny you could barely spot them till they were dead on the paper.
Perhaps invest in one of those or spray your plants with some neem oil or organic pyrithium when lights come on, leave it for a week and see if the new growth remains clean of bites, if not id wash your leaves down with soapy water then clean off with fresh water and hit them with another round of neem or pyrithium.
These are low impact solutions that are not likely to damage the plants leaves more than they already are but if you do have a bug problem it will most likely get rid of gnats and any small insects. Because my grow closet is in a spot to the outside environment I constantly am wiping away dead insects from my grow space that die from my preventative measures.
Better to clean insects every couple of days than to have them eat your plants till they are dead