We haven't actually looked for them yet. The frequency of the theoretical gravitational waves is also beyond what our current detectors can reasonably pick up.

We haven't actually looked for them yet. The frequency of the theoretical gravitational waves is also beyond what our current detectors can reasonably pick up. IIRC, a warp bubble a few km in diameter moving at <0.3c produces gravitational waves at a frequency just beyond what we can currently detect. Superluminal warp bubbles haven't been simulated, apparently that's beyond the computational resources available to this team. Don't expect a real detection any time soon. We'll need another generation or two of development on our detectors and to actually simulate a more realistic (small and superluminal) warp field. Personally, I think we'll find something once we build detectors good enough. I'm a firm believer that purely based on statistics there has to be someone else out there. I also choose to believe that superluminal travel is possible because the universe would be a much less interesting place otherwise.