@ubi-borealis All sorted and collected now! All Achievements too, thank you! And yes, as advised, it doesn't initially show on the world map, but just fast travel as close as you can then ride to Stenwege to find it in a chest
I'm also having the same issue which will block me getting the platinum. I stealthed through the camp in the main story arc so never actually killed the enemy carrying the wealth, and the marker just highlights an empty patch on the ground. Please fix!