This sentence is referring to the fact that the ring slipped away, abandoned Gollum who had protected it for many years because it wished to return to its former master Sauron. Sauron's spirit had been found wandering the wilderness by the witch-king of Angmar and was at this time being restored to his full power in his fortress at Dol Guldir by his servants the Nazgul.
In the caves of the misty mountains only evil creatures dwelt such as orcs, trolls and the Balrog of Morgoth, all creatures likely to answer Mordors call and any who found the ring would take it to Sauron.
The likelihood of a Hobbit ever entering these mountains was remote and Bilbo being their at precisely that moment to find the ring astronomical.
