Nah, I found him at the ferris wheel last. Which was funny because I was looking for him and couldn't find him, and then got up to use the bathroom, and when I sat back down I noticed I was looking right at him lol.
Anyway, he shows up once in each "section" of the City.
I have completed this sidequest and I have only met him 3 times before the ending of the quest. I think you need to open your map, check the sidequest info and you will trigger the next part of the sidequest.
For me he always appeared in the same spots in the same order.
First time is across from the court house. Second time is on the top of the ferris wheel. Third time is in direct view of where you land when you rescue Vicki Vale after her crash. Fourth time (which for me is always after I beat Strange I think) is on top of one of the buildings near where you enter the restricted area, south of the Ace Chemical building.
Then you put the pieces together on the map and the final spot is given to you.
I'm pretty sure how it works is that there are four locations, and he will be at a different spot depending on where you are in the story. For example he's supposed to be at the Ferris Wheel when you come out of the steel mill. And he's on a building near the broken highway after you save the reporter.
But let's say you didn't go to the Ferris Wheel, and instead you continued on with the main quest and saved the reporter. Then he will change to the highway, and once you meet him there he will go back to the Ferris Wheel.
If you do a quick search, there's tons of videos on it. "Watcher in the Wings Arkham City" should work to show you all of his locations.
I have the guide on hand. One is available across the courthouse after saving catwoman, thn, after finishing the steel mill for the first time, on the ferris wheel, then, after interrogating sharp, a building right aside where you interrogate him, and the final one, available after saving vale, is just west of the GCPD building, visible from where you save vicki vale to.
Madegame1000 is a pretty cool guy, eh makes posts and doesn't afraid of anything.