They appear to be bags laying on the ground, if you have labels 'always on' in the options they will have red names above them which makes them a lot easier to see. You could also collect night mushrooms to sell to a cook but they are worth less and also are hard to notice without labels.
The largest number of herbs (I think) are in the maps between Hydlaa and Ojaveda but there are some as well in the maps out the other end of Hydlaa. I hope that is general enough to not be considered a spoiler. If you find a patch you can just stand around until you fill your pack because they respawn after a minute or two.
