All about ferns
I suspect what you are seeing is the damage caused by slugs eating your fern fronds. The reason you don't see them is because they primarily feed at night or on cloudy days. Take a closer look at your ferns and see if there are any slime trails left from the slugs., Or if possible, go out at night and see if you can find them feeding on the ferns. Below is a UC Davis web site link which will provide you with more info about slugs, their habits and some possible solutions to your insect problem. Hope that helps!