Nice argument Chris! You put most of my thoughts much more succinctly than I could manage. Seems to me that, obviously depending on its size and how obvious it made itself (geostationary orbit etc.), we simply wouldn't be looking for it and so probably wouldn't notice it unless it happened to wander across a patch of sky that we were interested in. Which is why I suggested the amateurs because, as Chris says, there are a lot of them and most haven't got something particular that they are researching/controlling/studying, as so are more likely to stumble on it.
When you come to think of it, it's actually a bit unsettling
Although of course the real trick would be to arrive unnoticed. Once you are in orbit it would be pretty easy to hide but arrival... now there's the trick