A method of detecting sensor obstructions in a computing device includes: at an emitter, emitting a beam of light through a scan window toward a treaded surface; at an image sensor, for a sequence of positions of the computing device along the treaded surface: capturing a sequence of images corresponding to the sequence of positions, each image in the sequence having a first region and a second region; wherein the first regions depict a first subset of reflections of the beam of light originating from a first depth range; and wherein the second regions depict a second subset of the reflections originating from a second depth range; at a controller: receiving the sequence of images; determining, based on the second regions, whether an intensity of the second subset of the reflections exceeds a occlusion threshold; and when the determination is affirmative, generating an alert indicating obstruction of the scan window.