You have to look at it from when is a bag considered "checked?" Is it once the tag goes on, goes outside the jetway door, goes onto a baggage cart, or goes into the cargo hold? Some TSA people would argue once the tag goes on to make it simple. I can't quote you anything exactly because I don't think a reg would exist for this. If your bag had gone downstairs and gotten mixed with other checked bags, can you still get it back? Common sense will say your bag didn't pose a threat, but when's the absolute last point someone can call backsies on calling it "checked?"