We report on experiments performed within the Knudsen boundary layer of a low-pressure gas. The noninvasive probe we use is a suspended nanoelectromechanical string, which interacts with $4\mathrm{He}$ gas at cryogenic temperatures. When the pressure $P$ is decreased, a reduction of the damping force below molecular friction $\propto P$ had been first reported in Phys. Rev. Lett. 113, 136101 (2014) and never reproduced since. We demonstrate that this effect is independent of geometry, but dependent on temperature. Within the framework of kinetic theory, this reduction is interpreted as a rarefaction phenomenon, carried through the boundary layer by a deviation from the usual Maxwell-Boltzmann equilibrium distribution induced by surface scattering. Adsorbed atoms are shown to play a key role in the process, which explains why room temperature data fail to reproduce it.