Scene Context: Les begins to suspect the simulation not because of one glaring error, but due to subtle physical inconsistencies: the dead surface of the water, perfectly synchronized reactions, and the lack of bone conduction when striking metal.
Essence of the Phenomenon: Humans perceive sound not only through the air but also via vibrations transmitted through the bones of the skull. Therefore, a real physical strike against metal produces not just a "clean sound in the ears," but a somatic micro-vibration. Similarly, water in a real environment is rarely perfectly still and mirror-like due to ambient micro-vibrations.
Scientific Basis: Bone conduction is a real, thoroughly documented channel of auditory perception where cranial vibrations reach the cochlea. The absence of this physical feedback loop is exactly what would betray even a highly advanced sensory simulation.
Current Limitations: The more perfect the simulation, the more likely it is to be exposed not by massive rendering failures, but by the absence of ambient micro-physics—the subtle phenomena a living human body feels before the conscious mind even processes them.