Awhile ago, whenever my cat Jewel would sit on the windowsill outside, if Knuckles saw her, he would just start meowing nonstop and trying to get on the windowsill until we let Jewel inside.
Also, if Knuckles gets left outside and he wants in, he'll jump and hang on to the sill of the window in the door, and we'll just see his eyes peeking in lol