Have you seen your cats yowling at a stray cat that they see outside the window, pacing back and forth or even scratching the window trying to get out?
Do you think they are doing that because they want to be with their outdoor friends?
If your cats are strictly indoor cats and have no previous relationship with the outdoor cats, the act of frustration is due to their own territorial instinct. Cats enjoy their window view and are fascinated by what's outside. All the moving and living things from the outside keep them fantasizing about being outdoors and catching the squirrel they always wanted to.
Seeing another cat who has the freedom to do everything they have dreamed of gets them all riled up. If the stray cat starts urinating right in front of them, he is marking his territory and signaling the indoor cats that the outdoors is now his domain. Outdoor cats may even lounge by the window as if to say "I am outside and you are not." Imagine if you are told your dream has been taken away by another person, it'd be very frustrating. When cats see that happen, they can become vocal, agitated, antsy, trying arduously to get out.
Though not all cats react that way, some simply want to get a sniff out of the outdoor cat. They may start breathing heavily and even using their Jacobson's organ to help them detect any foreign scent. These mellower cats are being cautious about the new creature and trying to figure out the outdoor cat's status.
What should cat owners do about it? If your cats are looking really upset because of the outdoor cat, you can step in to help. First of all, cover the window blinds and move your cats to another room, distract them with toys and your attention. If your cats are pretty calm about it, you do not need to do anything and just let the nature take its course.