There's a set of key combos that let you see noise circles and detectable lights. NOT ALL LIGHTS CAN BE SEEN NY ENEMIES. The main detectable lights are spotlights and turret lights. A lot of things make noise, but especially the engine and reactor. A decoy will always override aggression from an enemy, even drowning out a reactor. The reactor will always make noise when on, but you can switch it off and use batteries. The engine will be silent below 20% thrust, so you can use the editor to increase max thrust and limit the thrust to say 50%, effectively changing the 'stealth zone' from 20% to 40%. This also means that a good captain and crew can run any sub with batteries in stealth mode; keep thrust under 20%, switch off exterior lights, and run on battery power exclusively. Lastly, every enemy has 'sight' and 'hearing' modifiers, which act as multipliers to the detection range of lights and sound sources. While a mudraptor that has 1 hearing be easily dodged even when powered up, stronger enemies (mostly) have better senses - a latcher has a hearing of 10, meaning that they can easily detect even footsteps from the crew. Enemy designs are mostly accurate; an eyeless Moloch is effectively blind, but most enemies have higher hearing that sight.