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.