The problem with fleet actions are that the harder SF we go, the closer to submarine battles and picket lines we get.
A starship is awfully hard to find in general. We have a celestially miniscule object that observers need to perform no small amount of searching for.
Thermographic searches are still line of sight based, and relatively short ranged.
Energy emissions are subject to a farraday cage type effect, and high-frequency lasers need to be interrupted to be detected.
Well, here is ANOTHER POINT OF VIEW:
Space is vast, but mostly empty. Space is also dark and cold; the average background temperature of space is 2-5 Kelvin. Ships with habitable life support sections, even with the engines off, will have a surface temperature of at least 200 to 250 Kelvin (Ice melts at 273 Kelvin). For a typical habitable section of a ship, the radiated heat signature is in the range of a few hundred kilowatts, which is generally detectable out to 30,000 km in under a day using a full spherical search pattern with a broad field IR band telescope with an aperture of 3 meters.
In addition to the waste heat generated by life support, a ship?s power generation system generates heat. A perfect Carnot heat engine produces 2 watts of waste heat for every watt of electricity it produces, where waste heat dissipation is free (like in an atmosphere). In space, waste heat has to be radiated. Minimizing radiator size (to make them retractable in combat, and to make them mass less) means running them at a higher temperature, which reduces the efficiency of the Carnot cycle. With a radiator roughly a 25m x 25m surface radiating from both sides at around 1600 K. Each radiator disposes of roughly 44 GJ of waste heat in 128 seconds, for a signature strength of roughly 340 megawatts, which is detectable (easily) out to around 10 light seconds (3 million kilometers) under the same conditions as the crew?s waste heat. (The distance from the Earth to the Sun is 500 light seconds, as a point of comparison.) Beyond that, for a ship using a reaction drive, even in cruise mode, it?s producing a minimum of a 340 gigawatt signature at about 2800 K, which gives a 1 day spherical search pattern ?guaranteed? detection radius of a bit over 1,000 light seconds, or roughly 2 AU.
Finally, any ship using a reaction drive reveals its mass by the correlation between observed rate of thrust and the temperature and brightness/mass spectroscopy of the exhaust plume.
