I had this problem a few times when people who live below me had a cat that was supposed to be indoor only but they later made him live outside and they would only bring him in at night. I live on the second floor and he managed to jump up onto my balcony and he was looking at my cats through the sliding glass door. I heard a lot of growling and hissing from my cats and I had to chase him away. It only happened a few times though. Now the people have moved away. My Pearl and Ziggy growl and hiss daily at both Alani & Blaze and it doesn't even faze these redheaded boys. Hopefully you'll be able to find something that will keep these cats away. Good luck.