So as I was finishing the second night, Mangle showed up at exactly 5:58 AM at the vent.  Fortunately, she didn't have enough time to kill me, however on the screen for 2nd night it doesn't actually show that Mangle is gonna be active that night. So, unless she's coded to show up at the very end as a "sneak peek" for the next night, I think its a bug and she was probably left active in the 2nd night by mistake.