As Friday mentioned it's the hypervigilance and cataloging all that is going around, but added to that it is following the pattern of what is natural or normal and what is not. For me it are out of pattern noises outside, especially when they are close to the house, and more so when I am going to sleep. Inside, a dripping faucet into a filling bowl or pot is murder, it is out of the normal and triggers all alerts. That said, so are the wild animals that scurry or scamper around the house at late or early hours, as they are being cautious and stealthy, fitting the pattern of a person. When I am full of anxiety or in deep ptsd triggered stages, any loud or unforeseen noise trips my wires and sends me to an alert phase.