The Thief's Whistle is a miscellaneous item in Ghost of a Tale. Tilo must find it for Whistle for a Thief, so that he, Gusto, and Fatale can escape Dwindling Heights Keep. Kerold Redwhiskers is said to have had one in his possession, but when Tilo talks to him in the Pirate costume, he reveals that he threw it down the drain and it is likely in the sewers. Once Tilo obtains the Sewer Key, he can find the whistle in the sewers, directly under Kerold's cell and alongside several florins.
Thief's Whistles are used to by members of the Thieves' Guild to communicate in secret.