How to find the missing car

Online is pretty buggy right now, it's entirely possible that what you experienced is one of these bugs. http://www.computerandvideogames.com/431870/rockstar-working-around-the-clock-to-resolve-gta-v-online-problems-errors-and-bugs/

The tracker puts a marker on the map, similar to the icon used for the Impound in offline player.

I believe the marker is called "Personal Vehicle" on the legend, so you should be able to find it fairly quickly.

Alternatively, if the marker isn't visible on the map, use your phone to call the insurance company and request a replacement.