I'm not sure if this is true, if the missile is coming head on. If it is made in a stealthy way, the "head" of the missile could obscure the engine area and make the plume invisible to the target. Having low RCS alongside with hidden plume could make a passively targetted missile relatively stealthy to the target.
Ofcourse, anyone with enough offset to the target would be able to easily detect the engine and thermal release but the target could be unaware.
Another method to hide it is if missile uses some kind of obstacle in order to accelerate behind, for example you fire your missiles and they accelerate when a moon or a planet break line of sight from the target to the missiles. This would make their acceleration burn invisible to the target and could get them very close without being noticed.
if you're curious about this topic, I highly recommend checking out the webpage linked in the sibling comment, "There Ain't No Stealth in Space" if you're curious about this topic.
tl;dr: it's exceedingly difficult to hide anything that consumes power in space. the rcs thrusters would either be too weak to achieve the delta v to hit the target, or the friction from the thrust against the nozzle would unstealth the missile. even the radiation from the onboard computer would likely stand out against the cosmic background. a missile like you describe could potentially evade current detection systems, but would almost certainly be detected by any civilization where ship-to-ship space combat is feasible. and not in a handwavy future tech way, but just from extrapolating current tech. I highly recommend checking out that page. it makes for a great read.
I've read it and mostly agree. The article even talks about the "solution" I mentioned (they call it cold plate). Stealth in space is impossible if you use constraints they used (solar system coverage with sensors and need to hide a ship with life support).
What I've talked more is about a missile. Missile can fly "cold", it could even be multistage so that after it's acceleration burn it can drop of a hot lower stage and continue to the target completely cold. If you add a cold plate and "terrain masking" you could effectively use a missile stealthily. Ofcourse if there is a full system sensor coverage this is not possible, but I've said that in my initial comment as well. You may be able to hide the heat from your target, but not from everyone.
Ofcourse, anyone with enough offset to the target would be able to easily detect the engine and thermal release but the target could be unaware.
Another method to hide it is if missile uses some kind of obstacle in order to accelerate behind, for example you fire your missiles and they accelerate when a moon or a planet break line of sight from the target to the missiles. This would make their acceleration burn invisible to the target and could get them very close without being noticed.