There's some evidence that that is *not* all he did; see my answer at [What did Smaug eat for 60 years?][1], for example.

Even so, Tolkien doesn't *need* to explicitly describe Smaug coming out from time to time in order for us to accept that he does come out.  He doesn't explicitly describe Aragorn going to the toilet, for example, but yet we can accept that Aragorn does do so.  Why shouldn't we accept that Smaug likewise comes out to get news and food too?


  [1]: http://scifi.stackexchange.com/questions/46395/what-did-smaug-eat-for-60-years