But your argument hinges entirely on the assumption that advanced civilizations will want to be found. There's nothing inevitable about that. Moreover, generalizing from the fact that we've sent broadcast probes in the past to "hyper advanced civilizations would send out von Neumman probes with broadcast beacons" is verifiably insane - I wouldn't even generalize from our decision to send out broadcasts in the past to what our species might do in the future with von Neumann probes. Just consider the heated debates over METI and how we're more or less not doing it. Now you want to take a tenuous prediction about ourselves and extend it to super advanced civilizations? What if advanced civs operate on a prime directive type basis and prefer to observe before intervening?

Granted, you can make this argument that it's unlikely that no civilization would send out von Neumann beacons. I actually would disagree - it seems to me that there are extremely strong incentives to not reveal yourself to other civilizations in this fashion.* Provided civilizations only send out a few waves of von Neumann probes, they may well choose to not send out beacon probes at any point -- especially if the probes are advanced enough to detect us without need for our finding and contacting the probes.

Of course, you might argue that at least one civilization deploying probes would attach beacons. But of course, you must realize that not every civilization is bound to deploy probes. Perhaps sufficiently advanced civilizations will be able to easily detect existing probe networks and just piggyback off of them. Or perhaps mega civilizations dislike having their territory infringed upon by other probes and simply destroy alien probes that approach their area. Perhaps civilizations don't attach beacons to their probes in fear that they may well be targeted for destruction.

My only point is this: it's extremely half-assed to say "surely von Neumann probes would feature beacons" and back that up with nothing beyond "wouldn't you do it?" and "if it's possible, it must have happened!"

*Partly because there's risk in revealing yourself to unidentified civilizations. But mainly because contaminating other civilizations' development process could impose serious costs, such as ruining your ability to observe certain alternative development paths. Essentially - a prime directive type reason.