The Lost Puff is a season 17 episode.
Paxton and Thomas start to play around after Sir Topham Hatt told them not to. Thomas ends up backing into Toby at a water tower. Water bursts out of it causing Thomas's firebox to go out. Paxton overhears something that Toby is saying to Stafford that Thomas had lost his puff and Paxton hears it, which leads to him trying to find it.