Guards cannot be called for certain cases in a cave.

[RainRain 591]

If a monster is outside the cave, you cannot call for guards if the guards are inside a cave.

If you are inside a cave, you cannot call for guards whatsoever, even if they are inside or outside.

If you are outside a cave, and guards are inside, you have to target a mob, leave the cave and call guards for them to attack it.