Cleo is a very stealthy cat. I suppose that it's something of a special cat skill, what with all that selection pressure on quiet stalking abilities. However, most of the cats I've known have had a very heavy tread, so you've usually been well aware of their location.
I opened the window in our bedroom the other day, then -- as I left the room -- I suddenly thought that I ought to check where Cleo was, and close the bedroom door so that she wouldn't try to climb out of the window. Sure enough, as I turned back into the room a second later, there she was on the window sill, half out of the window. Gulp.
I didn't want to frighten her, so I used my best 'talking-a-cat-down-from-the-ledge' soothing voice, and -- to my relief -- she came back in. I have no idea how she got past me so quickly and silently. She must have a tiny blue TARDIS or Star Trek transporter hidden somewhere. Since then, I've often found myself calling for her and looking in every corner, only to look down and find her sitting at my feet, looking up at me innocently with a "You called, Mithtreth?" expression on her face. She'd make an excellent Igor (if you're familiar with Terry Pratchett's books), minus the obviously sewn-on body parts, prominent lisp and surgical expertise, of course.