Why is my cat suddenly paranoid/hypersensitive/aggres… and how do I fix it?

One of my neighbors found a possum hiding behind the stove, it had come in via a loose door in the evening hours and had been in the house 3 days. Start checking the backs of cupboards with a flashlight - look for eyeshine or fur. Check behind the fridge and stove, anywhere an animal can cram in tight for shelter.