A Shadowdancer isn't simply hiding: she's using a Supernatural ability to not be seen while not having anything to hide behind – she's not actually in the shadow, she remains in plain sight but unseen. True Seeing will work on her.
A Ranger is simply hiding. His Extraordinary ability allows him to disappear into natural terrain while being observed, but it's a mundane hiding-in-the-grass he's doing. True Seeing won't work on him.
In a fictional sense, this should be sufficient. The Shadowdancer is using magic to conceal herself, while the ranger is using mundane skill. True Seeing penetrates magic to see things as they actually are and will show that the Shadowdancer is actually standing in plain sight in a shadow that has been magically moved/altered/detached.
Rules-wise, a bit more work is probably necessary. The description of True Seeing doesn't list anything that seems to cover this case, and an easy answer is simply that if it doesn't say it covers it, then it doesn't apply. However, this is a simple proof that it applies to the Shadowdancer:
- True Seeing "confer[s] on the subject the ability to see all things as they actually are."
- The two lists of things True Seeing are divided into magical and non-magical effects. The magical list is things it works on, the non-magical is things it doesn't work on.
- "Supernatural abilities are magical"
- True Seeing allows the subject to "sees the true form of polymorphed, changed, or transmuted things"
- There is no way to interpret a Shadowdancer's Hide In Plain Sight without it being a magical change.
- From all of the above, True Seeing should see a Shadowdancer using HIPS.
I think it was not uncommon for druids – and other mystical figures in folklore – to appear before others in magical disguise. Glamour, after all, is heavily associated with the fey, which are in turn tied to the natural world and the same sort of mythological background as druids. Further, as protectors of the natural world, druids have to assess potential “civilized” threats – appearing as a wanderer, sharing a campfire, gives you a good sense of whether the new people are going to be a problem.
Mostly, though, it just seems to play up the druids’ mystical angle. They know things, deep lore of the natural world but also about the comings and goings around them that they couldn’t possibly know. They have animals as eyes and ears but sometimes they need to see and hear for themselves. I think it is an appropriate, flavorful feature. And unlike most of the druid’s class features, I don’t think it’s overpowered.
Also, be careful about thinking too much about druids as being of or defending nature, because that was not really their historical or narrative role. The word “druid” itself means “oak-knower,” where oaks are symbols of all things ancient and deep. The druids were mystics, wise men, and priests. They drew their powers from the natural world, but much of their power was knowledge as much as it was magic (of course, at the time and in the myths, these were often the same thing). Any magic they had came from their knowledge of the nature of a world that had magic built into its very bones.
In role, they may have often been apart from society, but they were still very much human and very important to society. Their counsel was sought out in all things mystical and natural, which is to say everything that the common man did not understand. They were often highly political, kingmakers or rulers themselves as high priests. They were protectors of the old ways, which included human custom as much as it did the ancient natural world.
Best Answer
Ambiguous. The RAW simply doesn't specify, so all you can do is get a ruling from the GM.