This will unlock if you’ve already done the “Surveying the Property” mission detailed above. It turns out Gaston’s secret garden was walled off for a reason: it’s haunted! Head over to Gaston’s house to get started.
Go to the walled-in garden; you can just walk through the hidden door you opened last time after being invited into the Dutiel house. Mourn the tombstone and the apparition will request five pomegranates be planted in the garden to rest in peace. These can be tricky to find, but you have several houses. There is a pomegranate tree in Champs Les Sims; it’s in the front yard of the Pivert household, though depending on the day, you may not be able to harvest it (and when you can, it may not yield enough pomegranates at once). You can also buy them from the marketplace… in Egypt. However you acquire them, bring the pomegranates to Gaston when you’ve got them and he’ll ask you to check with the ghost and see if it needs anything else. Go back to the tombstone in the garden and mourn again.
This time, the apparition wants you to take his remains and place them next to his beloved’s, underneath the house. The remains will appear in your personal inventory; go down the stairs in the garden. You’ll see the glowing chest in a room near the stairs. Open it and insert the remains, then explain everything to Gaston for your reward.
The museum curator is having some issues with the security guards. Offer assistance to Estelle Fouchier to get started. She wants you to investigate the Egyptian section and find out why the guards keep quitting, so the museum is your next stop.
Go to the second floor and find the back-to-back sarcophaguses. View them, and then report back to Estelle. Next she wants you to snoop around the Chinese exibit, so head back to La Gallerie. This exhibit is near the entrance, and when you enter you’ll see a glowing hole in the wall. Inspect it to open a secret door. Before exploring any further, though, report back to Estelle and tell her about it. Naturally, she wants you to go back to the museum and see what’s in the secret passage. Head back and go through the newly opened door.
Follow the narrow hallway around the corner and go through the door and down the stairs to the room with two statues. There’s a fairly obvious secret door to one side of the stairs, which has a triangular carpet in front of it. Loot the treasure in the hidden room and go back to the room with the statues. Pull the torch and two panels will appear; push the statues onto them. Another secret door will reveal itself. Go through, loot the chest, and pull the statue forward to find a hole in the ground. Inspect it to reveal another panel, and push the statue in this room onto it. Head into the room with the large staircase, and walk past it and through the single door on the other side of the room.
The next area looks something like a maze; simply walk through the door that’s not unlocked. Up ahead, a wall will open up, revealing the room with the treasure you seek; however, as soon as you go through the door to loot the chests (avoiding the electricity traps to one side), it will block itself off. Go back out the door and through the newly unlocked door to the right, where you’ll find yourself in an area with three statues. Pull the furthest one out from the wall, and then push it onto the panel that appears.
Go out the door on the other side of the room, and loot the treasure chest; this will cause a foot panel to appear where the statue used to be. Step on it. This will cause the electricity traps in the next room to switch sides. Back out again, and go through the hallway with the visible wall traps, this time walking along the other side. Continue on to the room with the tombstone.
Read the sign on the wall to find out what happened to the curator, then mourn the grave. Inspect the hole to find the curator’s remains. You can check out the tomb for more goodies, then take the newly revealed exit out of the museum.