[2019] Red Boss Outline - When does this flash?

Either you entered the 40m mark or you 1) entered the 80m mark while another player was already at the 40m mark OR 2) you were between 80m and 40m and another player entered the 40m mark

Any of the above triggers the red warning flash on the boss icon once and the boss whisper will be more aggressive now when you go to dark sight.

So approaching the boss will always trigger the red flash eventually to tell you that you are close to the lair, however getting it at distance is an indicator that other players are around - possibly camping the boss.

No, that should not be the case, otherwise it defeats the whole purpose of being an intented benefit for those approaching an area over those already in the area - possibly in ambush. You are supposed to only get the warning once. Since you got it already when you entered, it shouldn't retrigger once someone else enters the 40m radius.

Correct, it would always flash again if they re-enter (but not leave) the 40m radius. Although, there is a certain delay to avoid this becoming too spammy, so it is only allowed to retrigger every X seconds.