Something Odd with Kill Walker Mission

[Jurassic Fart 1]

(playing through Ubisoft)
I've got almost 3,000 hours in this game, and I just discovered something very odd on this playthrough. Usually, when I tackle the mission to kill Walker, I kill all the enemies and that weak behemoth before going after the scumbag himself, but this time I did it differently. I ignored all the enemies as I flew my chopper directly to Walker's bunker, landed, and opened the door to go inside.

When I emerged triumphant, the local map was completely clear of enemies, and the behemoth was nowhere to be seen. Was this an isolated incident, or is it what always happens in this situation?

Oh, well, now I can scrounge resources without fear of molestation.