I've put together a map of almost every chest there is to find.

I use this to plan a route around the whole island for a single run.

https://imgur.com/a/KXdx8cY

Enjoy. Sneaker42

@ubi-borealis

• What did you do after collecting the key the first time (did you go to collect the treasure, or did you go to complete another quest)?

I've finished another operation or so. I had no idea the key was for a chest on a specific small island.
In the mean time I've finished the main game and now I can't open the chest.

• Can you please upload a copy of your affected save file to the cloud. This support article can show you how to do this.

The save file is synced to the cloud via Ubisoft Connect PC.

I've put together a map of almost every chest there is to find.

I use this to plan a route around the whole island for a single run.

https://imgur.com/a/KXdx8cY

Enjoy. Sneaker42

@ubi-borealis

• What did you do after collecting the key the first time (did you go to collect the treasure, or did you go to complete another quest)?

I've finished another operation or so. I had no idea the key was for a chest on a specific small island.
In the mean time I've finished the main game and now I can't open the chest.

• Can you please upload a copy of your affected save file to the cloud. This support article can show you how to do this.

The save file is synced to the cloud via Ubisoft Connect PC.