"Sneak into the rooms" is a mission objective in Sequence 13 / Memory 04: Raising the Dead
Walkthrough[edit]
Head either to the South. Be sure to use stealth to reduce your profile and be far less noticeable. Take your time if you want to go after the Optional Objective. If possible, snipe the Leaders to thin out the number of the Raiders below. Take your time and watch their patrol. There are a few opportunities to slip through. You can also use the cherry bomb to lure a nice number of them away as well.
When you make it into the Southern room, you will enter an Investigation zone. Head over to the left and interact with the specter there. This will reveal a series of symbols.