When Zuko, Iroh, and June bust into the abbey, the wide shot from outside the front gate shows the gate open. Then, in the next shot from inside, the shirshu smashes through the now closed gates.
Zuko and Iroh use June and her shirshu to help track down the avatar. The shirshu smells out Katara's necklace, making it go wherever Katara went. However, it brings them to the herbalist, when Katara never actually went there.