It’s definitely not exceptional, I currently have to build my structures in my single-player game anticipating possible approach by these guys because it’s so common.

The only possible indication you get there’s a raid incoming is that they get vomited out of the sky in large groups like the one you see at the gates of the fortress and start running in a random direction. Where they run is not going to be where they end up - they somehow found a way on top of the gigantic bridge/dam ruins once. I sneakily followed them and they just vanished into thin air and I thought nothing of it, until I got to my base and saw them in one huge ball just waiting on the bridge. At least I could kick all of them off.