If you just have an emitter symbol just inside the outer ring, (accompanied with a short BEEP when it first appears) this threat may or may not have detected you, but you can pick up their radar energy. (NON LETHAL THREAT)
If the emitter symbol is just inside the inner ring (accompanied by a repeating tone)this is a threat that has LOCKED on (probably with an STT lock, preparing for a SARH missile launch) (LETHAL THREAT)
If the symbol is well inside the inner circle,it has fired a missile (radar in guidance mode).
So, when a threat goes from non-lethal to LETHAL, the time for STEALTH is pretty much over and you better do whatever it takes to break lock or get a missile of your own in the AIR ASAP... Might as well turn on your radar to make sure you know where the guy is if you dont have good MSI track..
Of course the sneakiest ones are the IR missiles you never ever get warned about.