> We already sent probe out of our Solar System.
To give an idea of the scale involved, Voyager 1 is only 0.058% of the way to the nearest star system, and that took 45 years.
Granted it wasn't built for speed but given that the best hypothetical plans for a spaceship we have allow travel at ~.1c it'll take 50 years not counting acceleration / deceleration.
> Eventually probe will find planet with life. May be life is more abundant than we imagine now. May be not. Surely there will be protocols for the probe to investigate life. It's hard to imagine what those protocols would be. May be explore freely until certain technology advances are observed and then hide.
Even assuming that some set of automated probes have been travelling our galaxy for millions of years, and reached earth in the last few thousand years, where are they now? By what mechanism could they even be hiding? Every inch of the planet is being watched by satellites.
> My point is that developed civilization inevitably will expand its sphere of influence at speed limited by speed of light. Probably slower than that. But billion of years at 0.1c is still huge chunk of universe.
As an aside, I'm not sure you could meaningfully have one civilization spanning hundreds of millions of light years, the ends are just too spread out for there to be a single cohesive or web of cultures. Imagine if an email took longer than our entire species existence to reach you.
> Are we sure that no civilization born within this radius billion years ago?
I personally am certain that there are many other forms of life throughout our galaxy, and at least some of them must be technological.
As much as would like for it to be false, I also believe there is no realistic mechanism for travel between stars and we are all effectively doomed to stay in our little corner of creation, maybe spying on each other through telescopes, assuming we can even spot a neighbor.