Some more on sensors:
sublight mentioned different tech 'trees' that can be followed, all roughly equivalent, balanced differently but can be researched independantly. Applying this to sensors you can think of a few ways to detect stuff in space, some better at others at some tasks. Some with emitters and some passive-only.
First, instead of having detection or non-detection binary, we can vary it a little:
Presence - Artificial signatures detected in system, location and any further information unknown. Direction might be revealed.
Detection - Location known, but too uncertain to hit with weapons. Would start revealing details like size and type of vessel.
Lock - Location known with enough precision for weapons fire. More detailed scans are possible.
Stealth systems would prevent detection or even presence by masking any emissions while ECM systems might actually increase emission but decrease ability to obtain a lock. There are also varying degrees of locking your opponent which can factor into to-hit along with actual size. Knowing a 30m ship's location within 5m might be more than enough to fire on it, but a 2m ship within 5m might be a different story. Missiles fit with sensors might have an advantage in that they can be fired with only signal location detected and obtain a lock itself when it gets close.
LIDAR (active), Optical (passive) - possibly with optical a base tech for all
Thermal (passive) - bonus against populations and reaction mass propulsion drives
RADAR (active), EM (passive) - bonus effectiveness against shielded opponents
Gravity (passive) - weak sensor but powerful at high endgame levels, bonus against artificial gravity equipped ships and gravity-based propulsion systems
Radiation (passive) - bonus against vessels using fission or fusion power systems
Telepathic (passive) - Inbuilt in race, research has minor effect on effectiveness, cant be used to achieve weapons lock, effectiveness scales with population or crew
Laser (active) - Special for use with target painters to achieve weapons lock
Geological (active) - Deep planetary sensors, base tech
Also some more sci-fi sensor types:
Antiproton (active) - I imagine emitting antimatter and detecting even minute matter interactions
Magneton (active/passive) - Magnetic I think? From Star Trek
Tachyon (?) - Used a lot in Scifi
Spacetime (?) - Bonus against zero point power modules
As well as something truely exotic perhaps for the version of invaders we might use.
For sanity I suggest that all sensors operate faster than light. Simply makes programming much easier. Later additions could render sensors (most anyway) limited to lightspeed, but I doubt that it would really add much to gameplay if done.