> my several years old car will scream with alarms if I'm closing on an obstacle, and eventually even start handling the danger for me. I have to assume the same systems exist for a huge expensive warship. So not only was the AIS turned off - all collision alarms etc were too?
I don't know how the military systems work, but I have used civilian maritime systems like the AIS you mention, some of which are built by semi-military manufacturers. Let me tell you, AIS is no silver bullet.
AIS does indeed alert us when another ship is within a predefined radius. But it does not judge whether the distance is closing or widening. If a ship passes perpendicular to our course but less than the defined radius behind us, we get an alarm--exactly the same alarm as if it were directly in front of us and moving toward us. There is no feature like "alert 10 minutes before expected collision."
Also, AIS has a feature where you can send messages as a broadcast to all nearby ships. The message you send could be "SOS SINKING 12 SOULS" or it could be "SATURN MICKEY WELCOME TO SALVADOR PORT" (a real one I saw last week with the names changed). The alarm at receiving stations will sound the same in both cases.
In busy shipping lanes there are multiple of these messages received per day by passers by, and several obviously-not-dangerous proximity alerts at the very least.
You can imagine why people might be inclined to turn them off.