I wonder about one line in Drama
Detective Rhonda Boney: Has Amy got friends we can talk to?
Nick Dunne: Um...no, not really.
Detective Rhonda Boney: No friends? This whole town?
Nick Dunne: She was very close with my mom, before she passed away. And you know, we have a pretty serious homeless problem in our neighborhood. I think maybe you guys should check that out.
Officer James Gilpin: We'll look into that.
Detective Rhonda Boney: So you got to the bar around 11:00 today. Where were you before that? Just to cross that out.
Nick Dunne: Well, I was home. I left at 9:30, I got a cup of coffee, a newspaper. I went to Sawyer Beach and read the news.
Detective Rhonda Boney: Did you visit with anyone there?
Nick Dunne: Well, I mean I kind of go to Sawyer Beach for the solitude.
"Where were you before that? Just to cross that out."
In this line, what does "just to cross that out" mean?
and Besides what is "that" in between 'cross' and 'out'?
Please help me!