There seem to be 3 ethology logs in Emerald Vale, but I can't locate this last particular log and also can't find much about it online. The only clues I managed to find are that it's:
After looking going all around the area for half an hour, I finally found it:
It's directly south of the River Hideout marker, NW of the River Hideout south entrance & E-SE of the Volcanic Summit:
It is indeed a dark, hidden cave, and its entrance can only be seen from a specific direction, otherwise the surrounding rocks completely obscure it while blending into the environment. Here, the log's cave's entrance can be seen at the top right, and the place where the south entrance of the river hideout is, from which the river emerges, is marked by my companion move marker (arrow) on the left:
Hope this saves some folks some time. (I posted the question after already finding the place and knowing the answer.)
- There are indeed some Primals nearby. To the south of this log's cave, in a valley iirc. I believe I've encountered & killed this pack twice previously while going around, ignoring them the third time while searching for this cave, though when I came back after finally finding the cave, they seemed to have despawned.
- There is a dead Marauder atop the south entrance of the River Hideout with some decent loot. He can actually be seen in my third image.