I mean these horns:
Till the latest patch they were only making noise, but now triggering them causes machines to spawn (different type and quantity depending on location).
I highly recommend setting difficulty to guerilla and visitting Ängsnäs Manor
At least 8 Prototype Hunters with gas grenade launchers spawn every time the horn is triggered
There is normally a harvester there also calling in more hunters. You can run in some area and it will keep triggering more hunters. I use the area to increase region level.
These horns already called proto hunters before the Landfall update. On my last visit in Ängsnäs I was constantly fighting these hunters which spawned out of thin air sometimes right in front of me.
OK, maybe I was staying away from South Coast for too long