I've read that the 'You're being hunted' raid is one of the few, if not only raid that can kick off when you're out and about, the others require you to be near a certain number of 'base structures'. So I think that means before you 'unlock' that raid, when the games checks if a raid is going to occur, even if it decides one should, if you're not near a base it can't trigger one. But once you do unlock that raid...