in packages/geofire/src/GeoQuery.ts [524:531]
private _removeLocation(key: string, currentLocation?: Geopoint): void {
const locationDict = this._locationsTracked[key];
delete this._locationsTracked[key];
if (typeof locationDict !== 'undefined' && locationDict.isInQuery) {
const distanceFromCenter: number = (currentLocation) ? distanceBetween(currentLocation, this._center) : null;
this._fireCallbacksForKey('key_exited', key, currentLocation, distanceFromCenter);
}
}