I still have this quest active and when I enable tracking it says "read the letter you found". But I have no letter in the inventory (I read through all of them, even on "quest items" tab) and I cleared out every question mark on White Orchard map.

When I look at the video https://www.youtube.com/watch?v=Do-LA0FDW78 I even remember reading this letter some time earlier in the game (I just arrived to Velen) and actually clearing out the camp near the mill (and possibly every other camp). And I definitely killed the shield guy. I did this while randomly traversing through the map exploring question marks.

PS. it might be because I actually raided the camp before reading the note,