She says while reading the Skypiea poneglyph. "Poseidon....and it speaks of its location...who would had thought this dangerous thing still out here."
When you find the Alabasta one it will explain the ponyglyphs by Robin to the vivi's father. Like guy above said there are only 2 types. Ones with information and ones with locations.
Edit: Thanks for finding out about the poneglyph in the flashback, now I know Robin at least read 3. Just something that I'm adding up for later use.