Finally found the reverse way to get into the cave in Hill 137 by climbing the rocks.
Go end to the cliff jump on the small rocks next to the wall, do not use sprint.
When you get to the last large stone jump almost directly against the wall and you'll get on top. (it looks too high for the jump, but it is possible.)
Gives a very nice little flanking path to completely unaware enemies.
I'll make some images or video when I'm at home.