In the section on Fogmire, it states that if the PCs wander away from the old ruins, they'll find themselves wandering back into the ruins on the opposite from which they left within 1d4 hours.
After Bar-Iguras attack and Urol is kidnapped, we learn that the entrance to the shrine to Demogorgon is just under 500 feet into the jungle north of the old ruins.
So if the PCs head north to leave the old ruins prior to the Bar-Iguras attacks, what's to stop them from finding the temple?
Nothing is stopping them from wandering into the ruins. In fact, the point of the mysterious fog is to eventually drive the PCs toward the temple anyway.
