The Hobbit: The Desolation of Smaug : Why does Smaug leave the lair at all in the end?

The change in the movie made his flight pointless. In the book, just Bilbo was in and sneaked out, and Smaug went out and looked for the arkenstone.
In the movie he knew or thought they were still all in his lair and would likely take much of his treasure and he knew he was invulnerable, so why did he leave?

For the record, though, Bilbo did not find the Arkenstone in the book until after Smaug was dead. What he took was a cup; and Smaug did not fly off to Lake-town until after the second time Bilbo entered his lair.

"There are older and fouler things than Orcs in the deep places of the world." - Gandalf

Smaug didn't look for the Arkenstone. And in both book and movie he left his lair to torch Lake-town because Bilbo had carelessly let him guess that the Company had come from there, and as he couldn't get at the Company he reckoned the next best thing was to kill their friends.

In the movie it doesn't make sense.
He wouldn't leave his treasure horde that he has lorded over for 60 years to be plundered by dwarves during his assault on Lake Town.

Why not? He knew perfectly well that they couldn't cart it all away, or even run away for any significant distance, in the couple of hours it would take him to burn the town.

Yes, he knew that they couldn't cart it all away, but he stated that he would not "part with a single piece of it not one coin".

Thats the way I see it. Why would he leave the dwarves alone in his lair especially considering that he is a very greedy dragon? He also thinks he is invulnerable so in his view they can pull as many tricks with melting golden statues and the such, they would ultimately lose the fight.