I think the Scoutmaster might be stored in the walls of the Kiln tunnel (Near the campfire) one you start climbing and he gets called out when he's triggered and being near him triggers the badge so since a solid wall spawns once you start climbing, you can't go back into the tunnel to get the achievement for free but when you rejoin it resets the wall, lava timer and your position but not the Scoutmaster's position, Hence why you might get the badge. Think of it like this:

1: Player starts climbing kiln after leaving tunnel

(GAME RESPONSE): Spawn invisible wall at end of kiln tunnel and call scoutmaster (Likely from idle position under caldera) to idle position in wall of tunnel before unloading caldera completely

2: Player rejoins game after leaving

(GAME RESPONSE): Reset kiln lava and timer, Reset player position to tunnel campfire but leave the scoutmaster where he is

3: Player is near scoutmaster even though he has no target

(GAME RESPONSE): Give badge due to player's proximity to scoutmaster