I'm so sorry...that is always so disheartening.
As I always try to learn from each loss, I tend to ponder what might have happened especially if there is something I might want to watch for in others....but only if this helps (otherwise end this post for you now with a big hug)...
If interested...I can share what I've observed thus far, stating I am still always learning and not a poultry disease expert yet.
No sign of trauma or blood or feather loss indicates it was not likely a predator (unless you have snakes...but not likely in winter time)...Predators tend to be messy. Hawks tend to pick off one bird and eviscerate from the breast and entrails. Raccoons usually savage more than one in a flock, often picking off heads. Coyotes will carry off the whole bird leaving little trace other than a few feathers floating her and there....barring no signs of any of this...then environmental stress or disease is most likely.
For that, the body position can be somewhat telling. Environmental stress or weakness from slowly progressing wasting typically brings either a side laying posture with extension or a sitting position death. In slowly progressing disease, typical of organ failure, the birds typically slowly begin to weaken. Those will often succumb to cold, I've seen them die in the setting posture as they tend to want to sleep a lot more. I had one goofy but healthy bird, from what I could tell, die of apparent cold exposure this year...she curled up in front of the door opening...NOT in the warm cozy coop mind you...but right smack dab in front of the opening for some silly reason...on one of the absolute coldest nights this winter...25 degrees and strong blowing winds (wind chill to below teens?) which would have been swirling by that front door. I didn't check the inside of the coop that prior night, but even if I had seen her I would not have thought to move her, silly bird. I found her the next morning in a sitting posture, feet drawn under her in normal set, head bowed down to the straw, as if she had fallen asleep and succumbed to the cold.
Chicks that die from cold exposure (fall out of the nest or strand themselves away from momma's warmth), typically lay on their back or side and stretch to full extension.
One year I had Infectious Bronchitis go through the flock. I lost several to that illness. It started with the smallest little sneeze from one bird in the flock one night, and progressed over the week to where I had half my flock sniffling, snickering, and coughing. From that I lost 2. They succumbed and laid down and simply "gave up the ghost" by lying on their side with full leg extension as they give up their last breath.
In your case, you've had a bird die flat on her stomach with full leg extension. That could totally be any number of things or simply how the bird happened to shift into position (again only necrospsy would tell). However, whenever I hear of this kind of sudden death in that position, in a young pullet just before coming into lay, I ponder if the bird had been vaccinated for Marek's. Marek's is a viral infection that can produce either visceral (internal organ) tumors or neural tumors. With the visceral tumor, often no symptoms are present until the bird just drops dead. Young pullets just before lay are the most susceptible to Marek's. Marek's is everywhere and common, hence many like to vaccinate for it, but unfortunately the virus is mutating into different strains, so one vaccine may or may not help. Many breeders and hatcheries breed birds that have a natural resistance. Those with a weak resistance can succumb quickly. If vaccinating, it is standard practice to vaccinate at 1 day of age since it takes the virus several weeks to begin tumor growth. Older birds can be vaccinated, but it may not provide any benefit as again you are not preventing transmission or infection of the virus just stoppage of the tumor growth. If you have turkeys, the turkey strain of Marek's can produce a very minor illness in chickens that offers lifelong immunity to the more deadly chicken Marek's, providing the strain in your area has not mutated to far. (The original Marek's vaccines were based on the turkey strain.) Many small breeders don't vaccinate for Marek's as the industry only sells the vaccine in commercial quantities that must be used within 1 hour of mixture. I purposely got my bantam Cochins from a turkey farmer knowing that while no guarantee, I probably have a very high likelihood that these pullets will have gained a lifelong immunity to chicken Marek's.
If it were me, I would check to see if your feed store had the chicks vaccinated. Remember, that won't guarantee prevention of the illness, as the vaccination only prevents or slows the tumors that grow from the virus, but it will help the bird ward off the dangerous tumors that cause the paralysis or sudden death....or keep a few turkeys...again no guarantee as the strains can mutate, but it is an old farmer's trick.
If you would like a reading chart for illness, especially if you see any other symptoms in your flock, you can go to here:
http://www.apa-abayouthpoultryclub.... SYMPTOMS FOR DIAGNOSING POULTRY DISEASES.pdf
http://poultrykeeper.com/poultry-diseases/
and of course BYC has an excellent disease section with articles.
Lady of McCamley
I've lost a few over the years to unknown causes as well, mostly they've been young pullets, red faced, ready to lay and then they just turn up dead. Hadn't thought about Mareks, they are usually pretty asymptomatic, almost always in winter. With the number of birds that I have (all hatched and raised by me and not vaccinated) I would think that I'd lose more of them if it was Mareks. This year, I haven't lost any yet, but I have one that I'm keeping a close eye on. She's not doing well in the cold....for some reason just doesn't seem to be very energetic although she is eating, drinking and pooing normally (poo is a little loose, but not unusually so). She should also be on the verge of laying, we'll see how it turns out.
