How do you liberate an outpost undetected?

Liberating an outpost simply requires you to kill all highwaymen inside. This can be done in a lot of ways but the most recommended one is through stealth kills. You can liberate an outpost with guns blazing but that will alert everyone in the outpost and even force them to call in reinforcements, making your job harder. There are several factors you can consider:

Allied NPCs

Armed civilians, scouts, and New Edeners will be engaged by enemies if they walk within the perimeter. Sometimes, allied reinforcements will arrive as well and join the firefight and serve as a distraction. Take note that this happens randomly so don't rely on this happening on your liberation attempts. As long as you’re using a stealth/suppressed weapon, you can pick off the highwaymen while they’re shooting at the other allied NPCs without getting their attention.

Alarms

You can disable alarms by interacting with them directly or shooting them. If you want to maintain stealth, make sure to shoot them with a suppressed weapon or bow/slingshot. Take note that enemies will still notice if you shoot the alarm with them standing nearby. Alternatively, you can also set traps on them so they detonate and kill whoever attempts to use them.

Luring Predators

If you have baits and if there’s a hunting ground nearby, you can throw in a bait and hope that a local predator appears and gets their attention. Bears are the best since they can tank a lot of damage and capable of taking down high-rank highwaymen. While the highwaymen are busy dealing with the animal, you can use that chance to pick them off.

Proximity Mines

This explosive is important and very effective especially in higher-ranked outposts. Once the enemies are alerted, they'll always investigate the spot where you are hiding. That's why you don't stay in one spot as much as possible. You can however, leave proximity mines in front of gates, pathways, or your last position. You don't want your mines placed near each other since they'll have an almost instantaneous chain explosion. You can do this however if you're anticipating to lure a heavily armored enemy such as an elite enemy.

You have to be careful with the placement of the mines; don't put one near you until you're ready to move. Take note the enemies will also deploy proximity mines to deter stealthy liberations and sneak attacks; fortunately, you'll get a flashing warning on your screen and a very audible beeping noise if there's one nearby.

Environmental Hazards

Some outposts also have interactive objects that can help. For example, some outposts have a mortar emplacement in their perimeter that you can use to clear out enemies inside. However, using this will immediately blow your cover. Another outpost also has a pipe that you can turn to cause a pipe to blow up and spread fire, sending all guards to panic or even burn unfortunate enemies without blowing your cover. You can also set traps on MG emplacements and mortars to make them explode on anyone attempting to use them.

Guard Dogs

It’s okay to get seen by guard dogs. Unfortunately, you’ll have to kill quickly and silently before they can alert their owners. If you have Timber with you, he can scare away the dogs or kill them without raising an alarm. On higher difficulty, these dogs will be wearing proximity mines which will detonate at the closest enemy.

Flanking

If your companion gets seen, as long as you don’t get detected, you’ll still get the bonus. Sometimes, your companions will just bump into enemies and they won’t give a sh*t. This seems to be applicable to low-rank enemies only. Higher rank enemies will react differently; they'll be immediately hostile if they spot a companion and will actively pursue them. If you're willing to take the risk and possibly sacrifice your companion, you can order them to move to a position away from you so you can flank the enemies while they're distracted.

In other situations, you'll want your companions to attack a target. If your training camp facility is LV3, Timber and Horatio will be capable of pinning down and even killing elites.

Takedowns

Even if you get seen, as long as the detection gauge doesn’t completely fill up, you can still get the bonus. This is certainly applicable if you ambushed an enemy from cover and performed a takedown in front of him/her. He/she technically detected you but didn't have the chance to alert his/her allies. You also need to prioritize using takedowns against elites, unless you have an upgraded weapon capable of one-shotting them. They have the fastest detection and highest HP which makes them your main hurdle for maintaining your "undetected" status.