That's carrying a lot of assumptions. That there's not a probe lying somewhere in the solar system watching us, that aliens haven't found a better way to observe us than probes, that they even care about observing us in the first place. Take the last one for example - 400 billion stars in our galaxy alone. It's entirely possible that we're not important enough for advanced societies to care, the way you're not going to stop and mess with every random anthill you pass by.
Imagine a couple of tigers talking. "Someone says there are 9 billion humans in the world." "That makes no sense. Take the amount of good hunting habitats in the world, divide it by the amount of territory an adult male would need to call their own, and divide that by the number of humans. You would have multiple humans fighting for each chunk of territory, and marking their scent everywhere. Yet I don't smell any human urine so the question is...where is everyone?"
As an aside, why do people focus so much on self replicating probes? Assuming aliens did want to send out a probe fleet - why not simply mass produce probes in one system (or a handful of systems) and send each in turn to their destinations? Having to accelerate, decelerate, build a probe, have the new one accelerate, decelerate, build another probe, have this new one accelerate, decelerate, etc., seems like it would take a lot longer than simply sending a probe directly to its destination (no need to constantly decelerate along the way, no need to waste energy moving replication units, etc.).
That doesn't mean that they are optimal. The Fermi paradox does not care about optimal, just possible.
It's all speculation.
We have satellites and yet know very little of what's happening in other countries. We mistake their needs and intents all the time. We are regularly surprised of what they are capable or incapable to do. And yet those are identical being living in the same spot as us.
- they are interested in expanding;
- they didn't find a better solution to expand than going to other planets;
- their material structure is similar to us (they are physical, they have the same size/speed/time perception as use)
- the universe has no characteristics than we don't know about that could disallow us to perceive them (number of dimensions, or whatever)
- they emit some signals among the one we can perceive;
- they need and use energy as we do and as we understand it;
- they are not actively hiding;
- if we saw them we would realize what we saw.
It's like an ant assuming that because they can't smell pheromones from another hill, there is no life there.
Now, I'm for ocalm razor and I actually thing there is a strong probability that all those assumptions are true. But I don't know for sure that they are. They are just that: assumptions.
That's assuming that we'll get to that level by 22nd century (or ever).
I'm not so sure inter-stellar space travel and even space probing at that scale will come to be. Or even a colony as close as Mars -- for the reasons outlined pretty well here: http://www.ecosophia.net/terror-deep-time/
Not the only one. There may well be an armada of these probes on their way, but so far away in one of the billions of galaxies that the metric expansion of space means they will never reach us even at near-lightspeed - or may reach us long after we've become extinct. Or may miss our pale blue dot entirely. It's a freaking big universe.