There are also, as mentioned elsewhere, squirrels which inhabit this apartment complex. One would suspect that having a cat in the place would keep them out, so that leaving the door open wouldn't be a problem.
However, this seems to be a faulty hypothesis. It is worth pointing out that the squirrel caught in the above photograph did not just came from outside -- he came from our couch, where he was sitting when I noticed him. He fled to the doorway in front of the cat when I got up, but it is further worth noting that he remained there, nibbling at tracked-in leaves, while I retrieved the digital camera, turned it on, and took five flash photographs.
The cat, of course, was content to merely watch the squirrel with a vague lazy lack of curiousity, and has now gone back to sleep without having left the chair.