@rex - Good question! I will preface my answer by saying that we have only done some preliminary tests in this area. From our knowledge of how Sweep works, and how other LiDAR sensors work, I can say that as long as the retroreflector is designed to operate well at 900nm wavelengths (most will), Sweep will get very strong signals from these surfaces. If the reflectors are adjacent to surfaces that are difficult for LiDAR to see (like matte black or mirrored surfaces), it would be possible to distinguish them from the background using signal strength as a guide. I would not recommend this as a reliable tracking method though, as it is possible to get very good signal strength from non-retroreflective surfaces as well.