1) Get out cell phone
2) Prentend to call police
3) Describe subjects behind you as possible fugitives you saw in a Post Office Poster
You're going to do this because two guys are walking behind you???
As far as I can tell, neither of them have done anything menacing. Just two guys walking behind you as you leave the mall. Probably no big deal. I'm mean, this describes half the times I've been at a mall.
Stay aware, as always, and be prepared to act if it's not innocent behavior.