If you believe this to be likely reality, you're going to have to explain it in light of the Fermi Paradox because a galaxy-blanketing swarm of self-replicating machines is a lot of heat and mass to hide.
Wouldn't the explanation be that there never was any paradox, but rather we just hadn't noticed the alien probes yet? This is in contrast to the hypothetical scenario of finding microbial life on Mars, which as Nick Bostrom is fond of explaining [0], "would be very bad news" because it would imply the Great Filter is ahead of us.
Would it be possible not to notice them? The more probes there are, the more likely one of them happens across Earth. Interstellar space should be full of them. Unless we're talking about, like, one or two probes making a breadcrumb trail to Earth from wherever.
Well, I think it's a bit of a circular question :)
But it seems like one of the conclusions of the linked paper in the OP (which is trying to put parameters on this very question) is that it actually would be hard to notice them, for certain scales of probe (on the order of 1 meter), at least while they're in outer space:
> At a distance d from the Sun and the telescope, objects that are a meter in diameter and reflect a fraction a ≈ 10% of sunlight impinging on their surface would yield a flux of optical light of ∼ 0.2(d/1 AU)−2 nJy, well below the detection threshold of even the James Webb Space Telescope
But it goes on to suggest that we could notice them, even at sizes less than a meter, with radar technology, once they're in the atmosphere:
> In contrast, the radar signatures of a meter class object would be detectable with our deep space radars and space fence, much like IM2 was, out to beyond geosynchronous orbit at an altitude above 36,000 km. Such objects could also become optically detectable as they get close to Earth, especially if they create a fireball as a result of their friction with air.
But, such FLIR systems also produce a lot of false positives, i.e. actual everyday meteors in the atmosphere. So the challenge is narrowing down the data to find the most anomalous objects. And indeed that's what Loeb has done with the Galileo project, as he's identified one such object that landed a few years ago, and is chartering a ship to go try to find it in the Pacific Ocean. Estimating the chance of success of his quest is an exercise left to the reader, but we'll have to see what comes of it.
