Side-Missions -- Watcher in the Wings
This mysterious stranger appears four times in the game -- but if you miss one of the appearances you will then have to wait to do this during your Gameplay run -- so don't miss them!
The first time is outside of the Courthouse after you deal with saving Catwoman from Two-Face. He is on the building directly across the street from the Courthouse, on the roof.
The second time you encounter him is on top of the Ferris Wheel after you exit the Steel Mill for the first time.
Encountering the Mysterious Watcher
The third time that you encounter him is immediately after you interrogate Mayor Sharp -- look at the tallest building in the area and head up to its roof, you will find him up there and he will give you another symbol to scan.
The fourth symbol is found on the building directly to the west of the GCPD Building after you rescue reporter Vicki Vale -- just head up and the Watcher will burn the final symbol, and once you scan it the act will trigger your Batcomputer which you then use to enlarge and rotate the overlay of the four symbols until they lock together, revealing the 5th and final symbol location!
Head to the Church once you have completed this step and scan the 5th symbol on its side wall and the Watcher will appear and tell you of the prophecy -- which you do not really believe... Well, believe it, don't -- it really does not matter. This completes the Watcher Side-Mission and delivers the Achievement "Mystery Stalker" (15 GS) for your troubles -- so hey! Good on ya mates!