This kind of thing -- that humans can do today with current technology -- is why if an ET intelligence that could travel interstellar distances wanted to observe us we would never know unless they wanted us to know.

Their probes could be the size of sand grains, maybe even dust. Maybe not quite sophons, but not much better as far as our odds of finding anything. I suppose there would have to be something larger to receive signals from these things and send them back (because physics), but that could be hanging out somewhere we'd be unlikely to see it.

Yet another Fermi paradox answer: we are looking for big spacecraft when the universe is full of smart dust.