> Here's an amazing fact: The discoverer, Gennady Borisov, is an amateur astronomer who works as an engineer at the Sternberg Astronomical Institute. He makes his own telescopes to hunt for comets and has discovered seven of them along with several NEOs. He recently completed a new 0.65-meter telescope, the instrument he used to discover the new object.
Maybe a few more probes like this in suitable locations would give us a better chance of intercepting interesting stuff passing through the solar system.
That said, having dozens or hundreds of probes in random orbits would be advantageous, but still much more costly.
Parking one is also predictable in budgets where a random emergency launch isn't and will cost more than a planned launch just to get things done quickly.
 Excepting maybe a target that appears conveniently a the same L point.
So the advantage of already being in orbit is that you don't have to go through all of the logistics of getting the rocket to the pad, prepping it for flight, getting launch permission, etc... That's a difference of a couple of weeks.
By the time you have something like a nuclear rocket in space that has the requisite delta-v, there becomes no particular reason to station it around the sun, since you can get to the right vectors from anywhere in the system.
If then again Oumuamua was a probe traveling for millennia in search for alien life then there's no point of sending two in the same planetary system.
Imagine a civilization that is maybe 500 years more advanced than ours in space travel - perhaps they have nearly conquered their solar system. Instead of building ships the regular way, they instead carve up asteroids or other similar "space rocks" and attach engines to them in some way. But something went wrong - maybe the engines got stuck "on" or something, and the entire thing was launched out beyond their system, never to be seen by them again...
...and it floated for however many of our millenia before briefly tumbling through our system. To us, it looks like a rock - we can't even optically resolve it properly, it's too fast and too far away. In (fantasy) reality, though, on board is a bunch of technology that we could probably understand, that would allow us to "leap" to hands-on human exploration (and exploitation) of our own solar system.
It might be both a blessing and a curse.
Again - sheer fantasy - and most likely it was just a shard of rock and nothing more. But is it far fetched to think that there isn't "large scale" ships or probes of some nature just drifting through space, probably completely broken down and relatively inert, moving at sub-light speeds due to various reasons, but made out of rock because it's easy to get, provides relatively good protection from space, easy to modify, etc?
We humans have already flung tiny objects out of our solar system - I can believe that larger artifacts might be something that could "drift away" as well (then again, one would think that if a civilization got to that level - they'd be able to go out and retrieve it before it got too far away).
Maybe it'd make a good science fiction story - from both ends (the creation of the "ship", the "loss" after trying to retrieve it, then the journey and the eventual discovery of it by another civilization "far away" - and how it changes the discoverers socially, politically, and technologically).
At least some speculation on Oumuamua lead to the conclusion that it had unusually low mass for an object so large.
Oumuamua might well have reported something interesting 1000 years ago when it was 0.09 light years away. Maybe changes in the atmospheric chemistry related to human activity.
Also note C/2019 Q4 is approaching at about 50% faster than the previous. It looks like it's a comet with a tail... or is that a retrorocket?
Under the conclusion that space-born civilization would remain around stars. If we one day tap into other energy sources, ie whatever is powering dark energy, stars may become irrelevant or even awkward to be around. A civilization of kilometer-wide cities could be just beyond Pluto and still totally outside our perceptions.
Oumuamua as an alien probe sounds probable. Two of them in a span of two years, not so much. Besides contrary to Oumuamua this one has all the characteristics of a comet.
Presumably if you are watching a galaxy you are quite patient and a few 100,000 years is no big deal. With that kind of experience you've likely figured out what signature events are related to the rise of intelligence.
It's hard to speculate on the technology, sensors, and intelligence of the probe makers. Maybe the signature even to watch for is plankton, oxygen, and being in the Goldilocks zone (billions of years ago)? Changes in atmospheric chemistry related to clearing forests and cooking? (1000-10,000 years ago)? Emissions of radio waves (75 years)?
Whatever that threshold was, it might have been triggered in the past and a slow moving smaller ship was first (Oumuamua), now a few years later a somewhat faster (15%) larger ship is coming for a look.
Fun to think about anyways... at least we saw this one coming so anyone that wants a closer look will have the chance.
Not to mention there's likely some hierarchy of probes. The smallest ones relay to the bigger ones, the bigger ones can related to the huge ones, and the huge ones can report back home.
So maybe somewhere within 100 light years a factory started converting asteroids to probes and launching them at 100k miles an hour and lobbed a few our way. That would have been some 600,000 years ago.
Distance might be so difficult that a "ballistic approach" within a margin of error is the only practical plan.
How do you know that the ETs use a hierarchical filesystem?
But on the serious note why would you do that if your mission is only observation, data collection or maybe launching micro probes?
If an alien civilization 1M years ago saw earth and decided to shoot it for whatever reason, how much error would human activity have added? Seems like it would be tiny, but accuracy from such distances would also be huge, so could be 'enough'?.
Come to think of it, the chicxulub impact probably had a larger effect on our orbit than any human activity, and a far-away alien civ would have a hard time predicting impacts at such distance.
Would be interesting to make an interstellar warfare game that models the speed of light as a 'fog of war'. Projectiles are accelerated too fast for any control or deceleration systems to survive. Goal is to anticipate where opponents' assets will be in the future and move your assets away from where opponent will anticipate they will be. Could bombard your own planet to alter orbit at a cost to productive capacity. Kind of an orbital shuffleboard.
Presumably anyone sending probes that last even a few 100k years would be smart enough to have advanced sensors and notice if any adjustments are made. Even a very small of energy would correct a distant probe direction to encompass any possible earth position.
It will be interesting if the new 'Visitor' is shaped similarly to the previous. Maybe there's a robotic factory a 100 light years away and is turning asteroids into mostly passive sensors and lobbing them at likely looking stars at 100,000 mph to check to see if anything interesting is happening. So the launch would have been 600,000 years ago.
The important question on that idea now is: is this new object coming in along much the same trajectory as Oumuamua, or is it coming from a different point of origin?
A different origin would rule that out, but the same origin would be very suggestive of a relation between them.
Empty? You've seen stars in the night sky before surely?
Rare events also happen all the time; something being extremely improbable doesn't mean it will never happen.