Noise made by agents can be heard by nearby guards, causing them to investigate the sound. Noise can also be heard by Sound Bugs, in which case nearby guards become alerted.

All human guards can hear noise; the distance they can hear at depends only on how loud the noise is (i.e. the sound radius of whatever caused the noise), in particular different guards do not have different hearing ranges, Sound Bugs likewise. Drones have “no hearing” and cannot hear any noise.