The hunt for Cheng was on in 24: Live Another Day Season 1 Episode 11, as a voice from the past haunted both Jack and Audrey.
I loved the way Cheng said, Bauer when he saw Jack on that grainy video. It was as though he saw his worst enemy that just wouldn't die, and I guess that's probably close to the truth - but it's Cheng's own fault.
He had Jack for a year and chose to torture him instead of kill him. Looks like that decision has come back to haunt him.