See above...if the position is one that a person would normally be paid to do, it is considered work. You are correct in that the USCIS won't arbitrarily just start digging into a company. But all it takes is someone who has a gripe with the company to call the USCIS and say "I know there's someone at the company working for free and taking jobs". Next thing you know, the USCIS has their hands in your shorts...(so to speak).
In todays job market with all the resentment against immigrants "stealing jobs" and jobs moving offshore, this is a very real possibility. I researched this heavily when my EAD expired before I received a new one. I ended up taking a two week unpaid leave while I waited. I could have "volunteered" to keep working (read, work illegally), but didn't want to run the risk of it coming back to bite me in the ass.
Perhaps I'm overcautious, but I'd rather err on the side of caution than have a one way ticket back to Canada.
Regards
Mike