When Ultron notices the Watcher, the Watcher says
basking in the boundless silence of his universe ...
By that point in time, the universe has almost been destroyed, with only a few living beings left.
Dr. Strange also interacts with the Watcher only after his reality has crashed and he is the only living being left.
Does the "silence" of their respective universes help them in any way to recognise the Watcher, or was it just a coincidence?
Personally, I think it was their ability to see and hear beyond the physical world. They both had gained immense power, beyond that of a god. Which in turn gave them the ability to see and hear beyond space and time.
The Watcher probably masks himself in some regard, as to not be seen by anybody, including gods. In the show, it also looked like he was some how standing in-between worlds/timelines/universes, and looking in from the outside.
The narrative pacing of the final What If…? S1 Episode seems to imply the Watcher’s watching (his traditional observing and narrating) was sufficiently non passive to alert Infinity Ultron.
In the MCU, the quantumverse and fundamental theories of quantum mechanics have played an increasing role since Ant-Man and the Wasp, and they certainly are present within the concepts of the What If…? series.
This idea that the Watcher’s watching itself is a form of interference is associated to a fundamental theory of quantum physics, particularly the field of quantum cryptology, where the state of something may be altered by the mere act of observing its state. This paradox was first identified with experiments with light known as the “wave-particle duality.”
Wave-Particle Duality
Quantum mechanics, which physicists developed at the beginning of the 20th century, describes how light and matter behave at the miniscule atomic scale. They developed quantum mechanics to explain several observations that classical physics could not, such as the nature of light. One of the more common examples is consideration of the nature of light as wave or particle:
After Einstein realized that light waves also move as individual photons, Louis de Broglie suggested that electrons, and all matter, actually, must also move like waves somehow. A few years later, Schrödinger developed a wave equation that described the “quantum state” of a particle or group of particles. This equation was known immediately to be correct mathematically. But puzzlingly, it also described strange realities. For example, it states a particle could be in two places at the same time. When you measure it, though, it would only be found in one place. No one could explain what this meant in the everyday world.
In fact, to this day, no one completely agrees. Some physicists, like Hugh Everett, have suggested that the multiple possibilities dictated by Schrödinger’s equation actually predict multiple actualities—that is, many worlds, or the “multiverse” if you’re in the Marvel Cinematic Universe. The particle really is in both places, just in different universes, and they interfere with each other, but you can only directly measure one of them in your own reality.
From https://www.lsu.edu/research/news/2019/1023-quantumrealm.php
In this regard, the episode is offering the viewer meta-commentary on the nature of the Watcher’s passive act of watching (observing or narrating) as being acts in and of themselves insofar that it may simply be impossible for the Watcher to have ever kept his vow of non-interference, because “watching” may be interference to begin with.
