Stand out of the gathering in a crowd scene, disobey the director's instructions, escape the editing scissors and bingo. Fame!
To be fair, if you play out the scene, the smile can be interpreted as a concerned leer, or a resigned 'here we go again' expression. But it certainly sticks out in a film so painstakingly real.
Theory two: This was an intentional Nolan touch. There is always a joker in the pack who sees the funny side of things. "We are gonna get bombed and die anyway. Let's put a smile on that face."
Exaggerating it a bit more, probably this was a tribute to Heath Ledger's Joker in The Dark Knight. Why so serious? Eh? Life is fleeting.
Ironically, the scene featured prominently in Dunkirk's first teaser trailer. Now that the case of the smiling Dunkirk extra is going viral on the Internet, two things can alternately happen. An unemployed extra or 15 minutes of fame on a late night show. So much for celebrity status.