This Proficient Mam monolith (4/7) can be found south of Unuratu's home near the high walls of the UPPER CITY.
Reading the monolith reveals a clue: "A lone sentry stands guard over me and his harvest."
Selecting the monolith on the in-game map shows that the hidden cache is within a fairly small area to the west of the monolith itself.
Head down to that area, which contains many small garden plots guarded by a scarecrow made from charred branches.
The cache is buried just behind the scarecrow.