I've found the first two locations of the mystery stalker. The third is supposed to be just outside the Subway. However when I go to his location, there's a symbol already there, but no dude. Anyone have this problem?

By outside of the subway, you mean on the building? He appears there, across from the courtroom, on the building in front of the police station, and on the ferris wheel. You tried just scanning the symbol?

I wondered if it was location based. Not sure why I haven't noticed him about more.

Also, don't forget to throw a remote controlled batarang at him. It gets you an achievement.