But on certain occasions the report may indicate any number of planets... But the system map does not show them - why not??
When this happens, congratulations - you've discovered a truly unexplored, never-before-visited-by-anybody system.
In systems where other players have visited before(so they are "unexplored by you" but have been explored by other players), on your first visit doing the honk will reveal all of the previously-scanned planets. In a truly unexplored system, there are no such planets so nothing will show up on the system map, except the star and any really up-close planets that are within the 30 Ls passive scanner range.
Be aware that it's entirely possible for a system to only partially be explored - in which case the system map will likely seem to have large gaps and "voids" in it, where missing planets ought to be.