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 the name suggests, the caves are inhabited by a large number of yao guai.
To find the Sneak bobblehead, you will enter facing east, continue traveling generally eastward until you see yao guai run across your path from left to right (this is a scripted event, and the yao guai will not always be targetable in V.A.T.S. on either Playstation 3 or Xbox 360). If you have the Animal Friend perk, this event is different: the yao guai may or may not stand instead of rushing right to left. At this intersection, marked by a lantern on your left and an overturned bathtub on your right, turn left (north). Continue on this path to a Nuka-Cola Quantum on a table, and turn right (east) there. Continue on this path until reaching a cavern with two exits leading south. Follow either path generally southward, past a stop sign and a downed chain link fence, to the entrance into the yao guai den. Once inside the den, walk straight ahead to the central pillar (not the one in the small room with all the skeletons, keep going into large open area), and then head to the left. Immediately look for the stone ramp that leads along the eastern edge of the cavern, overlooking a crescent-shaped pool of water. At the base of the ramp (directly behind pillar with dead wastelander in the hole up high) is a metal box with the bobblehead standing on top of it. If you go up the ramp and drop into the water below, you will find the yao guai den note on a dead settler.
After the quest "Waters of Life" you will find an Enclave camp directly east of this position. They usually are all dead and can be found around the camp, 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. Also, a Vertibird has been known to land nearby (or any other mini-encounter involving Enclave soldier(s) may occur).
In the southern section of the yao guai tunnels there is a Grognak the Barbarian book along with a dead raider on the ledge directly in front of the one-way exit out of the yao guai den. You can get this by either placing the shopping carts in a stacked position so you can jump up to it or you can take the door in the southern end of the den to reach it.
A note called "yao guai den" can be found on a wastelander's corpse stating they were building a settlement in the den and had blocked it off from the rest of the tunnels. However the location of several raiders' bodies suggest the barrier was breached which may have accidentally let the yao guai in.