It's a mystery, but the lack of symptoms isn't all that uncommon. Chickens will hide weaknesses as long as they can - not only to avoid predators, but to keep their flockmates from bullying them to death.
Since your birds free-range, it wouldn't hurt to treat the whole flock for Coccidiosis, as a precaution. It's relatively inexpensive (Corid/Amprolium) and it's benign - it won't hurt them, even if they don't carry the organism. Coccidia are sneaky. They are present in all soils and can wreak havoc on a chicken's system way before any symptoms show up.
Since your birds free-range, it wouldn't hurt to treat the whole flock for Coccidiosis, as a precaution. It's relatively inexpensive (Corid/Amprolium) and it's benign - it won't hurt them, even if they don't carry the organism. Coccidia are sneaky. They are present in all soils and can wreak havoc on a chicken's system way before any symptoms show up.
