Having AIS on is mandatory, but in practice a lot of ships turn it off regardless. From shadow oil fleets laundering sanctioned oil to fishermen, fake or disabled AIS systems are hardly an exception.

I don't think Russia is trying to hide their sabotage, though. Even with AIS disabled, there's no way European intelligence agencies didn't know what ships were floating above these cables at the time they went down.

This was a warning, not a secret operation.