User blog comment:Mobo85/Hot Off the Presses..."Phineas and Ferb" Season 3 Starts March 4/@comment-1478533-20110215213400/@comment-1478533-20110215231454

The significance is in its later appearance, not in its first appearance (unless you're counting the split-second scene in the main theme). I'm guessing it will just be a random idea in that episode, and in the movie, it seems to become of some significance due to that teaser showing Perry and Phineas storming to Doof's tower with it.

"Chekhov's Gun is a literary technique whereby an element introduced early in the story becomes significant later on. For example, a character may find a mysterious necklace that turns out to be the power source to the Doomsday Device, but at the time of finding the object it does not seem important. "