Within the last week my Windows XP workstation has begun randomly losing focus from the active window. I've seen this in multiple applications including FireFox, Notepad, Putty, Visual Studio etc. There is no application stealing the focus, instead the active window simply goes inactive and there is no active window visible. In the taskbar the application I am working in is still highlighted - so that application is active but its window does not have the focus.
This is not focus-stealing feature you are thinking of that you can configure with Tweak UI. I checked that and its not enabled and anyway that is working the way its supposed to and looks different since its configured to make the grabbing application flash in the task bar.
This looks more like some other app runs, grabs the focus and then exits. The desktop is in the same state you have when you exit the active application.