Excuse Me...Your Fly is Down

This morning I went to the bank.  I had to wait outside because I got there a little early and the bank wasn't open yet.  As I was waiting this guy walks over and stands near the entrance.   As he walked over I noticed that his fly was completely down on the jeans he was wearing.  Then, we made eye contact to acknowledge each other's presence and I couldn't help but give a goofy smirk as I tried to hide my laughter.  All I could think about was "what should I say?...Excuse me, your fly is down."  

Do you tell a guy when his fly is down?  Yes.  But, what about when it is a complete stranger?  Do you tell him then?  What if he thinks I was looking at his junk when I wasn't.  What if he thinks I'm just trying to start casual conversation or if I like him?  So...do you tell a perfect stranger that their fly is down?  If I did, it would have been awkward, because it was only us standing outside the bank until the doors opened at the bank.  I decided not to tell him.  I assume some other stranger/passerby would mention it, but today it wasn't me.