I am not positive, but I think if one has done the Signal Disruption Tech research one gets an extra 4 hours tacked on to the advance attack warning. If not, then the Pandoran base is probably a good deal distance away. Citadels have a operating range that can exceed 3000 km (about the distance from Lisbon to Warsaw).
If there are no existing havens near the base that is getting attacked, that is why you are the target of this Pandoran base. Is your base in a mist-repelled area? If so, it’s even possible that the offending base could be in that area (if it existed before the mist was pushed away). Even other areas that have had the mist pushed away could be harboring a preexisting enemy base.
Make sure you have done the Pandoran Citadel research and re-scan areas around your base keeping in mind the operation range of Citadels.