They shouldn't get off during the day either except for a short time to eat and do their business. If they are getting off the nest and going off messing about like chickens do in the course if their day then they are not likely broody. Having a clutch of eggs in the nest isn't the only factor to induce broodiness, it may cause a hen to go broody every time you sit a bunch of eggs out if she is a very Broody hen. Others may only brood at certain times when the weather and their hormones tell them it's time to hatch babies and if I'd isn't the right time they may just let your clutch of eggs rot. I would set 10 or 12 eggs out of you have standard size hens. Some may be duds anyways.