Explanation: I just walked around in the facility when I suddenly heard 650's theme. When I turned around he was right there. I have not walked inside his containment cell, and the room that spawns in was a few rooms away.
Can it be repeated?: Not sure, it was very random.
Any other notes: Around half an hour before he aggroed to me I looked inside the observation cell (though not in the direction of 650). The theme didn't start until recently, so I doubt I aggroed to him then.