"By blowing into it, it generates frequencies only audible to the ears of dogs. You can call a dog with this whistle."
Dog Whistle is a key item which appears in the 2002 remake.
It can be used on the West terrace to call two cerberuses, one of which is carrying a replica of the Armor Key in its collar. A crumpled memo is found lying near the whistle, which explains how to use it. Once your character uses the whistle, they are given the option to discard it as they return inside.
The dog whistle is found in the Spencer Mansion and is placed near a chessboard in the 2F east study.