There are a lot of things they can die from with no symptoms. Some diseases are sneaky and occasionally have no symptoms; chickens are masters at not showing symptoms. Try listening outside the coop, or wherever they roost at night, to see if you hear rattly breathing or wheezing. When you're trying to find out if they have symptoms, don't stare at them; that's a guarantee they won't show any, because it reminds them, I guess, of what a predator would do before it eats them, so, of course, they don't want to show symptoms indicating weakness! Usually they will try not to show weakness around each other, either, as their friends may kill them. It's a defensive measure the flock will sometimes do, again to keep sick or injured chickens from attracting predators to the flock. You can also try just watching them from a distance out of the corner of your eye; pretend like you're looking at something else. Look for birds who are just sitting and staring, not moving around much during the day, or late to get down from a roost in the morning. Just so you know...you said it doesn't get hot there, but when it gets really hot here (high 90's) our chickens will occasionally have rattly breathing for a while when they go to roost. It seems to go away when it gets cooler.