How a laser ranger finder actually works on an odd surface
If the surface you reflect it of is very well polished and mirror-like, it won't actually work. Most surfaces however have some roughness to them which will result in at least some of the light being reflected back to the rangefinder.

In fact, a lot of LIDAR systems use rotating mirrors to scan. If rangefinders also worked on mirrors, that wouldn't be possible since the rangefinder would just detect its own mirror the entire time.