Say what?But how many bosses are going to have the audacity to hack into your CPU?
They don't need access to your computer to find out what screen name you tend to use. Certainly in a close community like where I live, all it'd take is for you to mention it to a friend who knows someone who works there and somehow mentions it in passing (believe me, this kind of thing happens) and it gets around and gets to your boss.
Gossip spreads very quickly in workplaces (if only relevant work-based communication spread so effectively...) - I happened to mention a job interview I had to a colleague who I thought I could trust and BAM, the whole organisation knew about it within about 2 hours. It wouldn't take much for a screen name to end up found-out.
Or you might slip up somewhere - I happened to have my screen name on my personal website (which I've changed now so that the screen name isn't mentioned). The domain name I used is the same one I use for email - the email address I put on my CV (Resume). From that they could deduce the website very easily, have a snoop around (I keep that site professional), find the screen name, google it... the rest is obvious.
And believe me, they *do* look. My boss commented on my website, so...
One other thing of interest, it'd be very trivial to any reasonably technically minded person to find out my home address.