Looted PDAs missing from inventory

[Thorus]

I have noticed while playing the game that when I loot some PDAs it prompt me to bring up my map through "P" but then show nothing on it. It will also be missing from my inventory so I can't even sell it after the fact. I thought this might be because it is tied to a stash that I already found and looted but I am not 100% sure.

Has anyone else had this issue, and if it is tied to stashes do you miss out on little bit of side lore if you loot stashes before you find their corresponding PDAs since some PDAs let you read a note that give a clue to the stash's location.