::snip
: Pippin mentions that he thought that treebeard had seen him earlier.

That's exactly my point. Only I believe that Treebeard did more than 'see' Gandalf. I think they had a long talk (even by Ent standards, LOL). Also, remember that Pippen's remark is in the context of saying that Treebeard seemed to be 'hanging around' the gate of Isengard when Gandalf rode up, as if he was expecting him. Indeed, Treebeard is the one accused by Pippen of "loitering" in this instance, not Gandalf, LOL! And neither Treebeard or Gandalf expressed surprise at seeing one another.

So why, you may ask, hadn't Treebeard already called the Entmoot before encountering Merry and Pippen, if he had already gotten an earful from Gandalf? Because we're talking about Ents, here. "Don't be hasty" is Treebeard's motto. Treebeard was mulling things over when he chanced across the hobbits.

We have evidence from Gandalf that supports this. Of the hill on which Gandalf meets with the others and where the hobbits had met Treebeard two days earlier, he says "(Treebeard) often comes here, especially when his mind is uneasy, and rumors of the world outside trouble him".

True, Gandalf does not say that 'his mind was uneasy' this particular time when Treebeard met Merry and Pippen, but I think it can be implied. And true, Treebeard probably had other sources of information about "the world outside" besides itinerant Istari.

But I think a strong case, albeit circumstantial, can be made that Gandalf was in Fangorn to rouse the Ents.

Note that, because of the timing, this idea does not conflict with the notion that Gandalf came to Fangorn to meet up with Aragorn and co. He could have been there to accomplish both tasks.