Why didn't Sauron sense Bilbo in Mirkwood?

I'm re-reading The Hobbit, and it says that Bilbo had on the ring for over a week while in Mirkwood, the same forest where Sauron was living. Why didn't he sense Bilbo and go to Thranduil's palace? Frodo seemed closest to discovery when he was wearing the Ring and close to Sauron. When Frodo put on the Ring in Mordor for a second, Sauron realized what had happened. And yet Bilbo was wearing it for a long period of time not very far away. For some reason Gollum under the mountains was not detectable, but surely Bilbo could have been. Unless... did Sauron vacate Dol Guldur before they were captured? I don't think it says exactly when that has happened, but I could be wrong.