I have managed to lose 'child' windows in Unity a few times.

For example, I started composing a new message in Thunderbird (so Thunderbird pops up the child window for composing the new message), and wanted to verify something in Wikipedia, so I clicked on the browser in the launcher (which was running in a different workspace) and it switched to that app/workspace. I completed my search, then clicked on Thunderbird's icon in the launcher to switch back. But nothing happened.

I manually went back to the workspace containing Thunderbird, but the 'compose message' window was gone. I found it, all by itself, in a 3rd workspace using the workspace switcher !!