"By blowing into it, it generates frequencies only audible to the ears of dogs. You can call a dog with this whistle."
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 2F east study of the Spencer Mansion.