Could what you are seeing be cecum (or sp. caecum) poo?
I copied and pasted this from crazy4daisy's
post :
"Chickens expel two different kinds of droppings. Regular intestinal droppings are firm, grayish-brown, and capped with white urine salts. Approximately every tenth dropping comes from blind pouches in the intestine, called ceca, where cellulose is digested by fermentation. Cecal droppings are runny, sometimes foamy, smellier than regular droppings, and light brown or sometimes greenish in color."
She copied it from her book, Storey's Guide to Raising Chickens by Gail Damerow.
Search for cecum, caecum, caecal, and cecal on this forum. You will find some pictures with which to compare.
The description that comes closest to our pullets is "melted chocolate."
Hope that is all it is.
p.s. I noticed it in our pullets at around 12 weeks of age as well. I don't know if it had to do with my becoming more observant, or if their foraging behaviour was broadened so as to create more of this type of dropping.