SmuggleCraft

This trophy is a little luck based and you really need to be paying attention so you don't miss it. Since the Mission Levels are randomly generated, there's no one-single level where you will find the Hidden Cave. The Hidden Cave is a shortcut off the main path, hidden within the mountain range. The opening leads to a cave that cuts through a portion of the main track. As soon as you enter one of these caves, the trophy will unlock.

I found mine on BOEI during a Delivery Side Mission called "Political Refugee". Here is the video showing the cave: