They could be ready to start their molt, it could be from the decreasing daylight. I have many non-hatchery type birds that quit for most of the winter. I got 11 eggs yesterday from 47 girls, all of laying age. I have many that were sitting, and many that are beginning molt. You could also worm them and check them for lice/mites.
I started worming them today, just to be on the safe side. I did earlier in the year, but I have added a "few" new chickens to the flock, and they may of may not have had any worms. I haven't seen any signs, but like you said, there aren't always signs. They also free range quite often, so, who knows where they may have gotten them.