Wow...nothing showing up on the trailcam...could it be that the animal is on the other side of the run and triggering the camera from a distance through the screen?
It's just strange that the camera is triggering but nothing is showing up.
Could you post a few of the trailcam images? I'd like...
From what you are describing...it sounds like it could be both raccoons and a fox. The raccoons are the ones eating the heads and evicerating the carcass. The fox would be the one taking the chickens with no trace of what happened, or leaving just feathers.
If you don't have a cage...