The Sight of Gandalf

[The Perky Ent]

There might be a thread like this, so I apologize if I'm wasting your time, but I have a question. In ROTK, when Gandalf asked Aragorn how they'd have any clue if Frodo was alive or not. Well, from that, you'd assume Gandalf's guess was as good as anybody else's. Then, after Pelennor, and Frodo and Sam have left Cirith Ungol and enter Gorgoroth, (when Gimli sits in the stewards chair ) Gandalf says

Frodo has passed beyond my sight.

That implies that Gandalf could sense Frodo and had something more than a guess. What's up with that? Does he like taunting Aragorn, or was it a "script spasm"? Just wondering