McChooky
Free Ranging
I had a similar problem and separated my heritage breeds from my leghornsWhat I would be looking for is something the other hens have in common that the victim does not; size, colouring, anything that makes her different from the others. With such a mixture it might be hard to see.
EDIT.
ISA brown/sex link I would say is the problem. You've got a high production breed in with a bunch of heritage breeds.