A Signal Whistle is a unique non-stacking item found only in the possession of Briggs and made use of in the Briggs boss fight early in Golden Sun: The Lost Age. Whereas most bosses' unique abilities manifest in the form of unique actions such as casting unique Psynergy and using monster skills, such as Star Magician's Mystic Call to summon additional enemies to join the battle, Briggs instead carries three of these items and uses them throughout his boss fight. In normal play, when Briggs uses a Signal Whistle, a whistling sound is heard, an array of blue music notes sprays out of him, and then a new Sea Fighter unit appears on his side of the battlefield, ready to battle from the next turn onward.
Because this counts as an item, however, it can be hacked into the player's inventory using, say, the CodeBreaker Advance code 82000B36 01C2 to make it appear in the eighth slot of Felix's inventory, or by using the Game Shark Advance code FBAD8D78 0B7B627B to make it appear in his first slot. Its inventory icon is actually that of a coin; it isn't meant to be found in normal play. As with many other items, Psynergy, and other properties whose in-game descriptions can't be normally viewed, the Signal Whistle's in-game description consists of three ???'s, but its Details description states that it is "Single-use", which it is. One would think that it could be stackable like other single-use items like Potions and Bramble Seeds, and that it could be sold to vendors; neither is the case with a Signal Whistle, which therefore feels much more like a key item when in someone's inventory (yet it can still be dropped).
When used in battle by an Adept, however, that which is most unusual about the item manifests; its audiovisual effect works, but the Sea Fighter appears on the enemy side ready to battle you just as if it were Briggs using the item on the enemy side. This occurs this way even though when you select the Signal Whistle for use in battle, you are prompted to select a target on the enemy side like you would using a Crystal Powder. When you defeat the Sea Fighter and the other normal enemies in battle, you get the additional EXP and coins for the Sea Fighter defeated.
Unlike other items in the games, the Signal Whistle is unobtainable in the debug room when looking through and possibly selecting the items to place into your inventory.