I can only suggest that you find her, catch her... or catch each one until you find the one that is having the problem... examine each individually and look for the problem... be sure to check them all over... including the vent.
There are sometimes traces or streaks of blood on an egg.. so, checking the vent would be advised. Look for "evidence of pecking" on the head, comb, etc.... and Check the legs for cuts or problems, or Scaly Leg Mites (the raised scales on the legs that can then bleed some).....
I am sure that others know more than I do and can input some ideas...
I just suggest that you find the one who is or was bleeding and give her a check over to see if she is still bleeding or not.
Best of luck.. Maybe use the Search here to search for "blood in nest"... or "hen bleeding" -- or "blood in coop, no injuries".... or even just "bleeding" -- you might find something helpful -- when you identify the hen and the problem... you can search the more specific issue.
Walk around, follow the chickens around.. check their poop.. watch the poop of the "buffs" until you see one with bloody poop.... (if there is blood in any poop then, Search here for "blood in poop".... be sure to Search "Normal Poop" first and go to the UK link provided that shows you what is normal for "blood" in the poop and what is not.)
Take care.
