At the start of chapter 13 in Resident Evil 4 Remake, you can purchase the Treasure Map from the Merchant for 5 Spinels who can be found sitting inside the cave to the right of the Wharf. This will display all the treasure locations on the map for the entire Island section including Chapter 16 as well as give you a small progression counter in the bottom right corner of the map. Although this will show you their general location, it won’t show you how to get them. Read below how to get all four treasures in Chapter 16.
Treasure 1 - Velvet Blue
Once you begin Chapter 16, exit the lab and boost Ashley up onto the upper platform on the left, then wait for her to drop down the ladder. Climb up and walk onto the bridge, then look up to find a container hanging from the ceiling above. Shoot it down and collect the Velvet Blue from the floor.
Treasure 2 - Iluminados Pendant
Progress through the chapter until you reach the Sanctuary, then follow the path around whilst paying close attention to the left wall. Eventually, you’ll notice a small hole you can crawl through. Here you’ll want to interact with the statue at the back of the area to obtain the Iluminados Pendant.
Treasure 3 - Gold Ingot
Continue following the main path along and head through the double doors, then continue hopping over the obstacles until you reach the Merchant Room. Here you’ll find a chest containing a Gold Ingot sitting directly outside.