I had two cockatiels that would fly around the apartment when I let them out of the cage. Kelly and Kasey were their names. Kelly was the female. Never had a problem. The guy I was dating at the time was a smoker, and I didn't permit him to smoke inside. So... he would meander out the front door and go down the hall past several other apartments and open a window that led out to the outside world. Enjoy his cigarette. Blowing smoke happily out the open window.
I was on the third floor, so through that window in the hallway you could look down at all the trees and other buildings in the immediate area. Nice view, on a nice sunny day. Well, when he left my apartment that one day, he neglected to tightly close the front door, and my birds were happily flying about.
Another neighbor came inside my apartment, and Kelly took this opportunity to escape out the front door and into the hallway, and... yes, you know where this is going... out the window in the hallway.
I was devastated. We looked everywhere for her when we got downstairs, but she was nowhere to be found. Her mate was depressed by the absence of his friend and I will never forget the incident.
Here is a picture of Kelly I drew on the computer.