You have a point. If something has been abandoned for centuries it's a pretty good assumption the ones who put it there arent coming back. It's the equivalent of wondering if the Egyptians are coming back to reclaim the pyramids.Even then, if something is abandoned for 1000 years, doesn’t finders keepers laws come into affect
The fault lies on the Thargoids for not taking this into account. Who knows how many times this had happened before and since the guardians.