I lived in an apt with my cats Clyde and Scooter. They are both strictly indoor cats. I had the window in the bathroom open once, I didn't notice that there was a small hole in the screen. I shut the window, and went to feed the two cats, but they were no where to be found.
I searched everywhere, finally I went in the bathroom, Clyde and Scooter were perched on the ledge outside the window. Both of them were really scared.
Now I am really careful when opening a window. It seems that no hole is too small to escape out of.