Rules of visibility
Visibility means showing the object on the galaxy map and being able to send a fleet to it.
It is not possible to send to or spy on an object that is not visible.

Any object can be seen from any other object up to 1000 diameters away. For example, a fleet with a diameter of 1 km and a station with a diameter of 50 km are 10 000 km apart: the fleet sees the station, but the station does not see the fleet.
Members of the same alliance have shared visibility and your friends only have shared visibility with you. Your friend's friend will not see your objects. For example, you are in an alliance and you have a friend who is not in an alliance. The friend does not have the visibility of your entire alliance.

All objects within 30 000 km of the centre of local coordinates are visible to all, as well as all satellites with active alstation, planetoid or psi field concentrator.

Each gravity gun shot makes the station visible for 24 hours to the owner of the object the gravity gun is firing at.
Visibility of an object in the Fleet panel reveals the visibility of a hidden satellite.
Visibility of the fleet by a sensor phalanx and using fleet sharing, does not reveal the visibility of the hidden station.
If a station is not visible in the Overview or galaxy map, it is out of visibility. It is possible to extend the line of vision by free flight of a fleet unit. To activate free flight send one unit to a high orbit and once in orbit go through the fleet panel to the fleet overview, highlight the unit and press the "free flight" button on the control panel.

When a fleet unit flying in "free flight" moves away from the centre of the local coordinates and out of visibility, that fleet will become invisible in the galaxy.