The secret room of Harold Manor was also inside the master's study of the main building.
In fact, most nobles were quite happy to place secret rooms in places like studies.
Because of privacy!
Compared to bedrooms frequented by many ladies, the master's study was the most private place in a noble's estate, where even the personal butler needed explicit permission to enter, aside from the estate owner.
As for security?
The very existence of the study's secret room, open and aboveboard, was enough to prove its security.
Newer noble families like the Lisop Family might still use mechanical machinery, but for a truly flourishing family like the Harold Family, secret techniques in machinery were standard.
Moreover, some places would use unique secret techniques.
Thus, even though many thieves knew where the most valuable things in the entire estate were, they dared not try their luck lightly.
After all, one only had one life.