My cat comes running home if he gets startled by a loud noise and he doesn't know what it is. It's enough to spook him, and he's not a particularly fearful cat. Some sort of motion sensor alarm might scare him off. I know you mentioned concerns with the motion sensor and the chickens tripping it. If they are not free range you can position it on the door to the coop facing out. Cats, dogs, and wildlife regularly trip my motion sensor light on the coop door, but it's rare that the chickens/ducks trip it. It just needs to be positioned facing out from the coop/run so that something approaching from a certain direction will trip it. You can play with positioning, motion sensors will have a certain range they can detect things from, height off the ground and the angle you position it will make a difference too.