I was on an exploration trek a while ago, and came up with a good way of spotting those pesky leftovers hiding out there somewhere:
simply zoom away from the star, with it targeted, and get about 1200ls away from it. This is beyond the orbits of most single star systems.
Then head back towards the star, looking for AM (anomalous movement). Your great speed out at these ranges (~100c), and view of the entire planet set as a whole, should make that missing planet apparent much easier.
If you mapped out the other orbits, the region to look will even be apparent, as it is the gap in the rings.
One time I was convinced the missing planet was off on a polar orbit (90 degrees off of the planetary plane), but so far that hasnt been the case. It was just a small moon like world.
So its reassuring that planetary mechanics of the starforge accurately results in a plane of planetary orbits, with only a few degrees variance possible.