Maybe some do all of that early stuff you talked about but you'd still end up fighting most of the dungeon. Do you have a motivation for each of the grunts such that you know which would take these side actions? Are you only talking about the named NPCs?
Just looked at an actual AP I'm running with an enemy camp. The camp is an oval but let's call it roughly 100ft by 100ft. There are 14 characters inside the gates. Every single one of them is a combatant. There are no cooks or maids or the like.
Of those 11 are stated as "fight to the death". The others aren't going to run away because what would their own people do to them if they ran from a fight? Unless they happen to be the last ones left, in which case the party still would have to deal with them for several rounds.
I think most people would deal with the problem at hand. There's 4 guys murdering people so that's what has to be immediately dealt with as quickly as possible. Splitting your force to deal with a "possible" side incursion from some possible alternate location doesn't make sense.
Me and my crew would all go and we would all kill the people I can see, then we'll spread out and look for more.
This is even a tactic in real life and in media. "I'll attack the front while you sneak in from the back". This works because it is understood that when you attack the front, people focus on the front.
Yes, in that situation it makes sense for all (or most of) 11 inhabitants to immediately attack the PCs.
Do the PCs enter the camp in broad daylight and announce themselves? They should be prepared for a result.
Do they enter the camp quietly and try to take out guards? Expect reinforcements shortly.
Do they enter the camp quietly at night? Maybe some enemies don't hear the battle because they are asleep. I'm not sure what structures are in this specific example, but the base DC will be zero before distance and barriers. Depending on conditions and levels, it is possible that not everyone hears the battle immediately.
Even if every person heard the combat... how many attack the PCs immediately? were they sleeping in their armor? Are they going to don their armor?
Again... every situation is different. there are scenarios where all enemies will immediately respond. In my last post, I was thinking of extremely large environments like castles, the encampment of an army, or very large dungeon where a whole community of beings live.
If the PCs attack a structure that has potentially hundreds of hostile creatures in it, it makes sense that not every single enemy is going to drop what they are doing and figure out what the sounds of a disturbance are. Most likely because in such large environments, the sound will not be heard, the creatures won't care or won't want to get involved directly unless it is their responsibility.
In the military, my duty was air craft maintenance. While on deployment, I was part of the designated task force to perform drills on defending our planes from attack. The scenario was basically if someone walked up to one our planes, planted an explosive and then blew it up. (Our aircraft were parked at the host country's civilian air port that was a 30 minute bus ride away from the main base and we only had a few buildings. Despite having probably 50-100 personnel on site away from the base, if an air plane was attacked our rapid response was to get 5 people to 5 designated zones in a certain amount of time. (several minutes).
So, in my real life experience, if enemies attacked multimillion dollar assets, the official military response is for almost every body to run away. Within several minutes, 1 person would show up to an area (to await reinforcements, which would take forever. They would have to be radioed an alert, then get ready for combat, then get into their vehicles and drive like 20 minutes to get where we are.
This is the type of situation I am describing.
Yes, if you have like 10 friends out in the woods and they getattacked, it makes sense for them all to immediately respond and the PCs should absolutely expect all the enemies gmto respond. If someone attacked the PCs, wouldn't they all respond? the answer is typically yes.