the first one i found by accidentally slipping out side the map after picking a white mushroom at location Yaulings Rot and picking up a lost letter
X-467.2
Y-451.1 approx.. dunno as i had to fast travel quick so i could get out without falling and dying
the next one was also by a lost letter at
X -279
Y-565.6 --again approx ..as after i got the lost letter and went back in the cave it would not let me see thru the walls again
same as the other location..funny how it happened twice today ....and both times in a cave that had a lost letter in them