Okay ... really?
Fine, I'll play along: What about Obi-Wan's "smirk" is inappropriate? He's "smirking" because Han's boasting is probably exaggerated, and he doesn't buy it. Han's trying to put one over on a crazy old fossil, but Obi-Wan's smarter than that.
And why is Han's smile inappropriate? He's not taking their "local trouble" seriously, but once Obi-Wan mentions the Empire he realizes that there's more to this old guy and whiny kid than he initially suspected.