I've seen the very same thing. I wondered why one particular guy I know managed to be relatively productive at his leisure with personal projects when development was also his FT job. He let it slip, effectively, that he was not so productive at work and even neglected it in favor of personal projects. This must be common practice in creating that mythos of the tireless programmer with a strong involvement in open-source.

Indeed, although it takes two to tango. Impossible to know the relationship between said person and their management. I take my inklings to start new things while employed as an indicator that I need to talk to my manager. Help doing more work should be a good request, and 'in a month' or so is an acceptable answer. If they don't help me out I start feeling restless and perhaps start that project on the weekend. If they don't respond to my progress then I start feeling unappreciated/underutilized, and express this. If it still goes nowhere, time to move on. Not moving on at this point is where the problems start, as you go into a reactionary mode and "aren't yourself" anymore.