Getting this, I followed Veilor's guide. However, I was missing one after getting all of them even though I was sure I had taken all of them.
To help find it I found this guide:
https://steamcommunity.com/sharedfiles/filedetails/?id=20837... (
https://major-gnuisance.github.io/util/lambdalocator.html)
Using your gamestate.txt-file it finds out which cache location it is that you are missing. That way you can go back and find the cache and hopefully the achievement will pop.
In my case, I was missing cache 12 in chapter "Water Hazard". I was 100% sure that I had already found it but I went back and the achievement popped. I don't know why it didn't count the first time but at least I got it.