When conducting a CSI: Poultry it helps to look over the body carefully for any breaks to the skin, any broken bones (neck? leg? wing?), and any parts missing. It also helps to look for feathers that fell off during the chase/struggle/capture. There's tell-tale patterns if the bird was chased, or smacked down by a raptor, or taken by a large strong animal. Look also for any prints in the dirt, bird feet with talons, or hand- and footprints of animals. And also check the feathers left on the bird to see if they're wet or disturbed by an animal's saliva.
The chicks you lost in the brush pile may have been taken by snakes. They can slide away unnoticed fairly easily. The snakes around here will sometimes catch & squeeze to death a chick that is too big for them to swallow. When I find a dead chick with its head & neck wet & slimy, but no other injuries, I know it was a snake that regurgitated the chick after it got to its shoulders. If you couldn't find the chicks they could have been taken by a large snake, or perhaps a feral cat bringing food to her litter.
I'm sorry for your frustration, it's so difficult to defend your flock from a predator you cannot identify. Your best general defense is to secure their sleeping quarters with strong sides, top & doors, and keep them in there at night. Next thing is to strengthen the wire around their run, maybe reinforcing the wire along the bottom few feet, maybe burying some of the wire. You can cover the top with netting or run some line across it and hang shiny things like old CDs, mirrors, & wind catchers. You may want to run some hot electric wire around the whole thing, or invest in one of those Night Guard lights.