In the last month I have lost 5, maybe 6 chickens. Lost 3 in one day. Don't see anything out of the ordinary. All eating and drinking. Do have rats in the coop. Could they be harming my birds?
I would send some of them in for a necropsy if they were mine and this was happening. We're spoiled here in CA with multiple places that will do that for very little money.
How old are you chickens? There may have been symptoms that went unnoticed such as diarrhea, puffing up, not eating, mites causing anemia, severe worm load, or getting into a toxin such as botulism or rat poison, respiratory symptoms, or crop or gizzard impactions. About once a week, it helps to pick them up, look them over, and feel of the breast bone for weight loss, look for lice and mites on skin, and look at droppings being left around the coop and run. Each day if possible watch their activities. If any don't come out of the coop in the morning, investigate why--that is how many of us first notice a sick chicken. Coccidiosis is a common killer of younger chickens, and it can kill over several days and weeks. Symptoms are lethargy, poor appetite, diarrhea--some with blood, puffing up or hunched look, and ruffled feathers. Most states in the US has a vet who will do necropsies on any chicken to find a cause of death. Here is a link for that: http://www.usaha.org/Portals/6/StateAnimalHealthOfficials.pdf