Even though this posting is older and Bob has returned, he was on what appeared to be a 3 month extended vacation. I believe he was out of the country staying at a beach resort, possibly the Azores.
Bob and Tom have been on the air for over 30 years now. Bob is 64, Tom is 61 and I bet both are looking at retirement at some point. Bob more so since he's older and Tom seems to be the main driving force behind the show, both on and off the air.
Since Bob's return, I don't believe he's working Mondays and they now have various add in personnel for the show. I'm sure someone is being groomed as a replacement (in name only) when he retires. First it was Pat McAfee, Indianapolis Colt's punter and place kicker, but not sure what happened there and why he would even be considered. Now it's comedian Scott Potasnik. I believe something will be announced by the end of 2015, at least for Bob.
I've been listening to Bob and Tom for nearly 10 years and love them. Unfortunately, they (like Gary Burbank) can't go on forever.