Yesterday afternoon, around 3 PM, I heard mockingbirds making a fuss
in my back yard.  When I looked out the kitchen window, I saw a
mid-sized tuxedo cat walking through my back yard, towards the house,
and two mockingbirds swooping down at him.  I think that they may
have even pecked him a time or two.  The noise continued for the next
three hours, as the cat had holed up on my back porch, where some
equipment leaning against the wall gave him some overhead protection.
Finally, a little after 6 PM, I went outside.  I called "Here kitty,
kitty, kitty", but the tuxedo kitty decided that two mockingbirds
plus a strange human was too much, and he moved off at a slow trot,
staying as close to the house as possible, with the birds continuing
to chase him.  I didn't see him or the birds again after that, so I
don't know if he was a feral, or a neighbor's cat that I hadn't seen
before (he wasn't wearing a collar).

As far as I could tell, Cinders never became aware he was outside.
There have been times in the past that I have found her staring out
the window at a visitor cat, but she doesn't make a fuss.
Apparently, since I have never let her go outside (too much traffic,
as I live on a through-road and close to a major highway), she only
thinks of the inside of the house as her territory.