Hriston
Dungeon Master of Middle-earth (He/him)
Incorrect. The party are not "hostile" before the barbarian attacks, and therefore the guards haven't noticed a hostile.
Hostile is a character's attitude in a social interaction in which the character opposes the goals of the other party and has nada to do with determining surprise. The question is whether a noticed creature poses a threat, which is a state of potentially being capable of causing harm and applies to most creatures, and certainly to all PCs.