Ariana Grande's gotta be wondering how an obsessed fan made it all the way to her front door before cops were called to track him down on her property. Law enforcement sources tell TMZ ... a man in his 20s showed up to the L.A. home of the singer. We're told he brazenly knocked on her door around 2 PM Saturday and asked for her. The property manager who answered the door told the intruder Ariana wasn't home.