Sorlin and Balin were Dwarves who had already been living here before the Lonely Mountain fell to Smaug's sudden attack.
And they used to be part of the upper ranks among the Dwarves.
The other Dwarves also roughly knew the way, but definitely not in as much detail as these two.
So once the two of them passed through the secret door, they moved about with great familiarity, as their memories of the Lonely Mountain were being stirred back to life.
According to the plan Sorlin had made together with Gandalf and the others, the Dwarves whose scent had already been remembered by the dragon could not go any deeper into the Lonely Mountain.
The dragon's senses were beyond extraordinary; they had best stay in this small, secluded area not far from the secret door, so as not to accidentally wake Smaug.
So only Balin, with the nosy Rong Buqiu and Bilbo in tow, continued deeper into the Lonely Mountain.
Originally, Sorlin had also wanted to come lead the way.
