One thing that is often hard to get right is figuring out why a dungeon or similar locale hasn't been looted before or dealt with. Often the reasons are contrived. There are several "interesting" places near my hometown, caves, quarries, etc. When I was young it felt like I was the first to discover it especially if none of my friends been there. But once I got older I found yeah people been tramping over these spots for many years.
This is not a big issue but I did think of something plausible that would make a dungeon unnoticed, even if it near a settlement. If the dungeon has a lot of magical creatures or content the combination produces a low level aura. The effect of this aura is that unless you have an express desire to visit the place you unconsciously ignore it. Planning to build a road next to the entrance. The surveyors report they have to swing it out a bit further because of "complication." Clearing brush along the hillside? "Oh that spot can be skipped.
To overcome this aura doesn't require a save or anything like that. You simply have to consciously and knowingly want to go there. For example finding a map and journal about how to enter the place is sufficient.
Tracking a beast or group who used the dungeon lair as an entrance is also sufficient.
I believe the Buffy tv series used a related reason as to why the inhabitants of Sunnydale were obvious to all the supernatural events that were going on.
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