Plugin code to gain access to all output window contents

I am writing a plugin for visual studio.

I would like to be able to detect and parse the text in all output windows that are created.
Some other plugins have a lot of information that is output in these windows and I would like to capture it and process it.

I've got the following code so far:

            _dte = (DTE)GetService(typeof(SDTE));
            _dteEvents = _dte.Events;
            _outputEvents = _dteEvents.OutputWindowEvents;
            _outputEvents.PaneUpdated += _outputEvents_PaneUpdated;
            _outputEvents.PaneClearing += _outputEvents_PaneClearing;
            _outputEvents.PaneAdded += _outputEvents_PaneAdded;

but for some plugins, these callbacks do not get triggered, and so there are some OutputWindowPanes that I cannot detect.
Do they use a different API somehow?