Originally Posted by FLIPADELPHIIA
I missed more than one on my playthrough and still got the achievement. You must have missed another one some where along the way
I did, whichever one it is that isn't on your list.
I even went back through the journal and ticked them off one at a time to see what I was missing but I had every single mystery in your guide except for the note on the hanging body.
I've since found out however that one of the collectibles available at the memorial building is able to be registered inifinitely so perhaps you got that more than once? Saves me having to worry about finding them all again on my second playthrough anyway.