I am in the starting system of Talison. What looks like a planet is displayed on the navigation screen in sector 80-0-2018 :
After a few jumps, I entered that sector and started looking for a planet with a ring. But nothing was to be found, and the planet was not displayed on the navigation screen anymore :
The explanation comes down to 3D space and you're zooming through objects that are above or below your sector plane. In Evochron, objects can be above or below your location in 3D space, including by full sectors. So when you zoom in, you can zoom through them so that they no longer appear on the map when the zoom range distance is below the distance of those objects above or below your position. You can flip the map around to the rear view mode to gauge the elevation distance as you zoom in/out.