I think the most logical, plausible, and scientific possibility would have to be the Alpha Centauri system.
First, Alpha Centuari (the name of the star) is not very far away at all. According to NASA, roughly 23 times the distance from Earth to the Sun, or
about the same distance from Venus to Earth. Think about that. We've sent a number of probes around Venus and they've gotten there in a matter of a
few years. So think: If there IS a civilization that is more advanced than we are, and has much better propulsion systems than we do, and they exsist
on a planet in the Alpha Centauri system, it would be very possible for them to send a probe or ship to Earth in a reasonable amount of time. The
Sirius system is several light years away, and each light year is about 6 trillion miles or so...so it would be VERY unlikely to see anything from
that system reach here in any reasonable amount of time.
Oh, and here's another interesting snippet from NASA's site:
"By an exciting coincidence, Alpha Centauri A is the same type of star as our Sun, causing many to speculate that it might contain planets that
Sounds like a real candidate to me.