The door to the outside that Chase comes in is left wide open. That's how the three Walkers following him got in. But the doorway was never shut or secured by anyone afterwards.
B.J.: (over the radio, not knowing that Chase just killed a Walker in the dark) What did I tell you? Easy breezy.
Harris: (with a scratch off lottery ticket in his hand) Do you believe this shit. I've been playing my whole life and now I just hit the five grand. (throws it at a dead body) Here ya go.
Airs only on the AMC official website for The Walking Dead.