AJ didn't leave town, he was killed while in the hospital with a broken back. Originally, it looked like Michael was going to have been the killer, but it turned out to be Dr. Thomas. The last time AJ came back to GH, he kidnapped Michael and faked his death, planning to take him out of the country. He and Jason eventually had a confrontation in the Q mansion and that was how AJ ended up in the hospital. It turned out AJ had once hired Dr. Thomas to kill Jason, and AJ was blackmailing him...which is why Dr. Thomas killed him. AJ has resented Jason for awhile, with the fact that Jason helped raise his son a major factor. Hmmm. Actually, if my guess is right, I think I got a major point wrong: Monica knows AJ is alive because she hid him away somewhere...but it was to keep Jason safe, not AJ :lol!