If there are only a few sites where seeds are being produced then u r cool. It is not uncommon to have a seed appear. Nature is not predictable no matter how much genetic engineering we attempt. Take a couple/few seeds of ur harvest and plant it up again and see. Lots of reasons can cause seed production especially in artificial environments. Chill if most of ur crop has big buds all over it!! Ensure you thoroughly clean down ur grow area for next time