Baron buff gives away location

Was watching a stream on Twitch (LoLGeranimo) and noticed something. He was in a bush about to ambush a Lucian. However, the baron buff gave away his location (not exactly but let Lucian know that someone was nearby). Easy way to change this is to not allow the baron buff be applied by stealthed/hidden champions.

EDIT: It was the baron buffing the minions that gave it away. Just realized I didn't specify how it gave it away ^_^