Yes, that's possible, it still feels bad though (not to mention that most of the time sysadmins can still access my not exactly private machine) - why not work at a company that values me as a resource?
I can sit bored (with a couple of hours worth tracking in the timesheet software) for a month or two, but then I usually start looking for something interesting.
You're missing the point. If the rogue employee is already taking advantage, what's to stop them from doing so in a manner that also defrauds the company of any proof of ownership over a pet project?
If you're doing it on your private machine over which you have full control then how can anyone prove it was done during work hours?