Yao Guai Tunnels

The Yao Guai Tunnels are located due west of the Charnel House, and northwest of Smith Casey's Garage. The entrance is a chain-link fence door in a rocky outcrop. As expected, the caves are inhabited by a number of Yao Guai.

Since all of the enemies here are animals, the Animal Friend perk will cause them to ignore you (though you can still kill them for the XP.)

Loot
There is a Pre-War Book on the picnic table just as you enter the Tunnels, and a random weapon behind the picnic table, in the cabinet.

In the southern section of the Yao Guai Tunnels there is a Grognak the Barbarian book along with a dead Raider on a high ledge on the south wall (accessible as you exit from the "Yao Guai Den").

There is also a bottle of Nuka-Cola Quantum sitting on a picnic table in the northern part of the Tunnel map.

Floating in the pool of water next to the Vault Boy bobblehead is a settler's corpse containing a note about setting up camp in the tunnels.

As soon as you enter the tunnels there is a picnic table to the right with a wall of sandbags behind it. Jump over that wall and there is an assault rifle in the cabinet.

Bobblehead Location
To find the Sneak bobblehead, turn north at the first major intersection and follow the tunnels to the door to the Yao Guai Den. Once inside the Den, walk straight ahead to the central pillar, and then head to the left. Immediately look for the natural stone ramp that leads along the eastern edge of the cavern, overlooking a crescent-shaped pool of water. At the base of the ramp is a Metal Box with the bobblehead standing on top of it.

If you search the corpses of the wastelanders on the water, you will find a note which explains what happened here in tunnels

Watch out for bugs- you can get stuck in between rocks on the edges of the cave so save your game before exploring the edges.

Enclave
After the quest "Waters of Life" you will find an Enclave camp directly east of this position. They are all dead, with clear signs of a struggle all around. They may have been studying the Yao Guai for ways to control them similar to the Enclave Deathclaws, or may have simply not known about the Yao Guai den when they set up the outpost.

Three Enclave soldiers should make their way to the camp shortly after you have arrived.

There is a scavenger who has somehow tamed a Yao Guai that visits the area, it is possible that his Yao Guai killed them all.