I walk my ferret with my dog probably 2 times a day. Today we were walking past a bush in our normal walking route and her and my dog both reacted to it. She did something strange that I have never seen a ferret do. She arched her back, puffed her tail, and did not move. It closely resembled when a cat gets upset. She did not want to move so I came and picked her up. Even after picking her up she was still almost stunned. We had to walk past it one more time and again this happened. I’m not to concerned, more just curious.
You are quite right, it’s the same hyper alert stance that frightened cat will use. There was probably something under that bush potentially scary. A feral cat or a fox maybe.