In an interview the president said he was using the term wiretapping as a general statement for surveillance, not just phone monitoring. While they are still looking into it, it would make sense if he was being monitored. Obama's administration gave permission to NSA to tap Angela Merkel in 2010. Those intelligence departments (CIA, DIA, FBI, NSA) have conducted surveillance on many people including celebrities like John Lennon over the years. It would not be a big leap to think Obama would want to monitor his political adversary. When you look at leaked information from whistle-blowers you learn to "appreciate" how little privacy the average person has.
Honestly, even if they do not find evidence that does not mean it didn't happen. If I was the incumbent president, and conducting surveillance on the succeeding president, I would destroy any of that information before they took office knowing that they will have full access to all branches of intelligence.