Assuming one civilisation within a sphere 1000ly in diameter, that makes 16000 in our galaxy alone!
It's much weirder than that; after all, there's been life on Earth for at least 3Bn years, but space-going life for only the past 57 of them! Stack it all up: if this is some sort of life-created megastructure then it's not just evidence of life, but of presumptively-multicellular tool-building life that can get off its planet and survive in an intensely hostile environment that it didn't coevolve with.
With an estimated 100Bn stars in our own galaxy alone, this would suggest we can expect to see on the order of a million interplanetary-capable life forms in the process of carrying out mega-engineering projects, and yet none of them have come calling on us? Fermi Paradox calls bullshit.
For a hot and fast civilisation like us to exist for serious amounts of time in space, it's going to have to adapt to living in space --- because, as you say, it's intensely hostile. One of the key adaptations is to stop being hot and fast. If you can live on small energy budgets you're way more likely to survive. If you live slowly then travel becomes much easier, as well as costing less.
But if you become a cold and slow civilisation, inner solar systems aren't going to be very hospitable --- hot, radioactive, space is so bent that travel is enormously difficult, everything's at the bottom of cripplingly expensive gravity wells... why bother? The gas giants are rich, but are even more expensive (to get from low orbit around one of the moons of Jupiter to low orbit around another takes about 10 km/s. You can launch from the surface of Earth for that). But further out... the Kuiper and Oort clouds are peaceful, cheap, hospitable, and chock full of light elements with interesting (if slow) chemistry.
Estimates of the Oort cloud density predict that there'll be a 20km body every 30 light seconds or so, and that there's about five Earth masses in Sol's Oort cloud alone. And Oort clouds of neighbouring stars overlap...
So I'm expecting any successful civilisation to start colonising the spaces between the stars, rather than the star systems themselves.
In this model, our own solar system could already be colonised, and we'd never know. They'd probably have noticed our radio transmissions by now, but it may not have occurred to them to answer yet. They may not bother; if we survive long enough to adapt to space, we'll be out there soon. Or they may not be able, depending on their energy budget. Or maybe they are, and we just haven't noticed. (New Horizons has a 12W transmitter and we would have to be looking in exactly the right direction to spot it. And it's way closer than the Oort Cloud.)
Of course, this doesn't explain why our hypothetical civilisation has built a hypothetical megastructure around this star. I wouldn't know why they'd want to do that (except for noting in passing that interstellar travel for hot-and-fast creatures like us basically requires a Dyson sphere for the energy requirements).
I'm planning a book. Can you tell?
Before you start writing your book, you might want to familiarize yourself with the prior art in the field: start with the short story "Swarm" by Bruce Sterling, then the novel "Permanence" by Karl Schroeder, "Blindsight" by Peter Watts, and maybe "Diaspora" by Greg Egan. That's assuming that by "planning a book" you're thinking in terms of fiction rather than SETI-oriented non-fic, but even if you're writing non-fic it's worth noting that we SF authors have been all over the topic for decades. (As for non-fic, the prior art goes all the way back to J. D. Bernal's "The World, The Flesh, and The Devil", if not further: I don't read Russian but I gather Nikolai Federov wrote on the subject, as did Konstantin Tsiolkovsky.)
Here's the paper that crystallised the idea for me:
http://arxiv.org/pdf/1201.2687v1.pdf
Also, yes, fiction --- I'm not enough of a scholar for non-fic.
It's also possible that more advanced civilizations are paying attention but are waiting for us to get over our kill-it-with-fire stage in response to large-scale visitation :)
Fermi Paradox assumes ET would like to communicate, or be allowed to be detected in some way.
That's a very big assumption, given how many unknown unknowns exist out there.
It's worth noting that KIC 8462852 is an F3 star very similar to our own, which is a G2.