Where There's Muck There's Brass

Once you've got the metal detector (and unlocked Detectorist) then as you explore the map you'll see notifications in the bottom-right when your metal detector has detected something to dig. Use the onscreen instructions to use the metal detector to find the cache, after 10 regular caches the achievement will pop.

So long as you get the metal detector relatively early, and follow up on most of the pings you get whilst playing, this should be easy to unlock without any special effort during a regular playthrough.