If you live in a house with a zillion smoke/CO2 detectors (we have one in each bedroom and one per floor in addition to that), you may have seen this behavior. When the batteries get low, the detectors beep. This drives my guys crazy! The most insecure (Daisy) immediately starts shaking and wants to crawl up my shoulder. Even Mr Gatsby was getting freaked out, and he usually does not get scared about anything.
These beeps are impossible to triangulate and figure out which battery is low. Yesterday I felt so sorry for the guys, as I had to leave them when they were freaked out to go get more batteries. Then, when replacing them, you are supposed to press the "test" button whenever you replace a battery, and they got all freaked out again.
Next time, I have to replace then when they are out for a walk. I wish I knew why they hate those beeps so much...