So, say we discover a method of FTL. However, anyone or anything that makes use of this FTL is unable to be observed by the outside world.
For example.
In the year 3000 a ship leaves Earth headed 6 light years away at twice the speed of light. A side effect of this FTL prevents outsiders from observing them. From the Earth's point of view the ship vanished into thin air. The ship arrives at it's destination, drops out of FTL, and can now be observed again. From the Earth's point of view, in the year 3009 their telescopes will see you suddenly appear at the destination 9 years after you left(3 to account for travel and 6 for the light to arrive back to Earth). However, after 1 year at the destination the ship decided to head home. They go into FTL and are once again un-observable. They arrive back at Earth in the year 3007, 2 years before their arrival at the destination is observable on Earth.
So, my question is does any of this in any way break the law of causality and effect, and if so how does it do so? As I understand it the fact that they can't be observed actually going faster than light that should allow them to get around causality.
And when I say they can't be observed during FTL to outsiders, I mean that completely. No heat escapes, no energy, no signals, no light, nothing at all.
Thank you very much