When its 2 war vessels, i’m more inclined to belive it has to do with collecting SIGINT just like their Air Force at the US border and other EU countries. They approach at high speed directly, dont respond to communications, and ‘listen’ to all of the defense systems that are deployed at them(radar, IR, missile tracking, etc), and observing any interceptors to understand their response protocol.
Then they just bank hard, stay outside of the unauthorized air space and give their data to military intelligence to chew on.
