[Vertigo]

Due to the vastness of space (blah, blah) it is quite possible that it wouldn't be detected immediately, just depends where people are looking. On the whole, most orbital monitoring will be looking the other way; ie. most satellites are looking down at the planet to spot things happening (like missile launches etc). When looking into nearby space we tend to be looking at things we already know are there. After all, bearing in mind that for the most part we are not expecting aliens to arrive on our doorstep, so where would anything unknown come from that hasn't first been picked up by monitoring launches from the surface? In other words I doubt there is any kind of constant monitoring for unexpected arrivals in orbit. So I would say, depending on just how big your UFO is, it would be perfectly possible for it to be first discovered by an amateur astronomer, you'd me amazed just how many things out there are first noticed by amateurs.