hi there...this actually happens a lot with kitties. Several things could be going on ...
- Cats commonly will hide when there has been an environmental change. By this i mean...house guests, additions to the home (2 or 4 legged), food change, etc.
- Cats also hide because they could be sick. Being 7 years old, illnesses that come to mind are early kidney disease, infections, heart disease, among other metabolic diseases. This can only be determined by your vet by physical exam and blood work.
- You make a good observation with the outdoor cat. just the fact that there is a cat outside could also cause your cat to be nervous and scared. Since the cat on the outside has an upper respiratory infection, don't let your cat and the one outside come into contact with each other.
I hope this helps. Id go see your vet just to be sure there are no underlying problems.
M. Vitucci MS, DVM