There is plenty of scope for special warheads just using the robot handbook.Decoy missile.
A missile that matches the sensor signature of the firing ship, until a detailed sensor scan is made either blip could be the ship.
A triple turret with three of these would give a pirate ship a bit of trouble since there are now four targets and they have to scan each one - how many sensor stations does the corsair have?
In this case however you can see shape visually and categorise the thermal signature of the source long before you can interrogate the transponder, so I think it would be spotted. There are doubtless missile borne decoy measures possibly like firing a massive cloud of multi-frequency jamming chaff between you and the pirate but they would probably only offer temporary relief but it might force them to choose another attack vector to reacquire. You might even be able to deploy a huge reflective balloon that resembled a vessel and they might foil (ha ha ha ha) sensors

You could maybe have some missile style drones that conform to you and put out a thermal signature that created an overall picture that made you look like a larger ship.
It might be better to just keep it vague to avoid nit picking and just say decoy missiles using some clever space "magic" apply a negative DM to sensor rolls. I am sure there are some secret projects going on in the worlds militaries at the moment to solve exactly this sort of problem, just because it is not in the public domain doesn't exist an d therefore isn't actually magic.