Ok, I see how this was possible and it's fixed in the next update. Basically the base was less than 75 meters from the monument, and my code only checks 65 meters away. An oversight.
You can fix this in the meantime by editing the DangerousTreasures.cs plugin file
Change
if (!IsLayerBlocked(position, eventRadius, blockedMask))
to
if (!IsLayerBlocked(position, eventRadius + 25f, blockedMask))