EDIT:
He found more Unown Blocks in other forests as well and they seem to spawn pretty often in his server. I tried to search some Forest Biomes in my server but no Unown Blocks. He also found Unown Blocks in Birch Forest.
And the most weird thing, he was walking in a Unown Block forest and suddenly a Hidden Pressure Plate popped up as an entity. :S
EDIT2:
He found Unown Blocks in Taiga Biome as well. Also, he went again to the place where the Hidden Pressure Plate popped up and another one popped up again. :S
Then he broke all the blocks below that area but nothing special.