The Bentley's Medallion is in the Old Crypt which connects to the south Munbury Cave entrance by a tunnel you have to discover. You have to enter the Crypt from the Munbury Cave side. If you have not discovered the Old Crypt through the other entrance already you may need to first. If you have already discovered the Old Crypt, try raising your perception and looking in the south entrance area some more.
The secret passage in the Emerine Golem part of the Munbury Cave is probably a WIP that you cannot enter yet, at least according to the most experienced players on the Patreon page Comments section. I tend to agree with them.