In Batman: Arkham City, you eventually acquire the ability to spot informants for Riddler trophies (they're highlighted in green while in detective mode). I've noticed that these informants keep popping up in the same spots after interrogating them earlier.
Do they always show in similar spots, and do they stop spawning at some point after you've gotten all the information about an area out of them? Or will they just keep spawning? I've only probably located (not gathered) about 75% of the Riddler trophies on the map at this point.