Besides the two Holocrons that are easily found in Dagobah, there are also two hidden ones. The first one is found at the big tree with two eyes with a short cave. Go near that cave, and look to the right. You will find the first hidden Holocron behind some branches. The second hidden Holocron can be found when you see the big rock to the left of your path. Use Force Grip to move the rock to find the Holocron.