I saw the other cat walking outside my sliding glass door, so I got my cat, Kitty, and opened the door to see what she would do. My cat is an indoor cat and this is probably the farthest she's ever gone outdoors. I knew she was territorial/aggressive like this, but I'd never seen her chase another cat outside. After Kitty went back inside, she would not stop sniffing me and everthing in the apartment.