Depends on what you mean by FTL. Most Scifi canons take FTL to mean "ignores relativity", like Star Trek and Star Wars. In this context someone using a relativ

Depends on what you mean by FTL. Most Scifi canons take FTL to mean "ignores relativity", like Star Trek and Star Wars. In this context someone using a relativistic viewpoint would simply see the ship disappear, then appear somewhere else before they left (using a huge telescope or whatever and calculating the time difference). The results of return trip are totally based on the interactions of FTL with relativity as we know it. It could be totally ridiculous. Imagine the a ship here leaves for Alpha Centauri (AC) at hyperspeed or warp 9. For this discussion lets assume that the travel takes 10 minutes. But for light it would take 5 years (rounding). This functionally would be travel 5 years into the past. So earthbound observatories would see the ship right now, because it left the Falcon/Enterprise 5 years ago from a relativistic stance. Then the ship could return and see its own light given off at alpha centauri? What does this look like from the viewpoint of observatories on Alpha Centauri(AC)? Thinking about it, it would have to look the same. By definition FTL is faster than light, but light is attached to time. So the trip from Sol to AC must either go back 5 years or otherwise reconcile our 5 year clock skew. I don't see how any reconciliation mechanic makes any sense. But yeah you are right that a ship that simply accelerates so much that its riders think and measure their own speed as faster than light are observed by outsiders as hugging light speed.