Great work on the game. It's really starting to move on from just concepts to a... well, game. Seriously, good job.
Anyway I've come up with some in universe explanations for sensors and just wanted to know what you think. That said lets get to some world building.
Sensors: Sensors were invented alongside FTL travel originally as a way to detect jump points. This is done by detecting hyper spatial displacement or to be more precise, the displacement between 3D space and 4D hyper space( the phase). The smaller the displacement the greater the signature. As stated earlier, originally in the early days of space travel the rudimentary sensors could only detect the massive signatures of jump points (areas of space where displacement is 0) but it was soon realised that as every energy body leaves a signature sensors can theoretically detect anything made of matter. After jump points the size of a signature is based on mass, velocity, and radiated energy(flux) though not in any order. In order the largest material signatures are stars, planets, large moons and asteroids, large ships, small moons and asteroids, medium/small ships. The exception to this are ships with cloaking devices which synthetically increase hyper space displacement. (currently only phase ships have the ability to shift hyper space displacement)
Transponders: With the discovery that tachyons act near identically to photons it wasn't long before a form of FTL radar was developed. Low frequency tachyons (as they interact most with matter) are fired periodically in all directions before ricocheting off a material object and returning as a ping. During the federation it became law for all transponders to fire in a binary pattern containing information of the transponding ship, as to be easily detectable by other ships. This custom has outlived the federation as it easily distinguishes rouge and outlaw ships. Though transponders do extend the range of sensors they still struggle to find medium to small ships not using transponders due to their small size and manoeuvrability relative to a planet. they also cannot detect anything in hyperspace (phase ships) as tachyons cannot naturally cross dimensions.
Idea: when a ship that is not using a transponder and is in the range of a ship that is but not it's sensor range it will send out periodic pings.
Phase ships: Ships have devices that can manipulate hyper space displacement by creating/annihilating vast amounts of dark energy.
Tachyons: Every particle has its antiparticle, photons are no different. Anti-photons (commonly known as tachyons) are not particles that travel faster than light (as is commonly believed). They act almost identically to photons, travelling at C and being unacceleratable, instead accelerating space-time. Tachyons do the same but accelerate space-time in the negative. This gives tachyons the appearance of travelling backwards in time arriving at their destination, anywhere in the universe, instantaneously.