12 Apr 2017 16 Apr 2017
These look like the logo on the shirt that Rex wears. A skull with flames on the side. If you go close by one, it will show up on your map.
In Pagoda, almost right where that area starts on the map after the street carnival, go along near the ocean & there is stairs leading down to a small dock. This is right near the pig cannon if that helps.
Under the bridge/overpass near the airport, close to where the overpass goes into the tunnel.
Apollo Island, behind the big hangers (near the free run).
Top of the dam in Bluebell. I recommend waiting till you have all disguises, as you might as well get the other collectibles for 100% while you're up there.
Bottom of the Auburn Bay Bridge (Golden Gate Bridge). It's on the strut that is closer to Auburn. You will have to run up a blue & white ramp.
Edit: If the achievement isn't popping, Davehx suggested the following. "Press your guide button and go to the achievement app on the xbox and load up your achievements for the game. I had a couple where the achievement notification didn't pop on the screen but had registered in there, and one where it popped as I entered the achievement app".