Recognizing that any Alcubierre drive system requires exotic negative energy density matter that, as far as we can tell, does not exist, if somebody managed to find such stuff in the quantities necessary to fuel a working Alcubierre drive, would it have an observable signature that could be detected from afar?
I know that there is a huge energy release at the end of the trip. But I can find little information predicting what it would look like.
If there is some predictable pattern that would be detectable from a great range, then it would seem a good target to seek out. We wouldn’t have to build an Alcubierre warp drive to prove it is possible: We would just need to look for specific kinds of flashes of energy to discover that a far more advanced civilization has already figured it out. And, in so doing, discover that such a civilization exists.