While playing Batman: Arkham City then you have probably seen the Riddler trophies with nothing but two 4-digit numbers next to them. You cannot scan the numbers and there is nothing nearby to interact with. The solution to these Riddles is that the numbers are Coordinates.
On the bottom right of your map then there is a compass, next to the compass are two 4-digit numbers. Go to the coordinates given to you and there will be something to interact with. Once you interact with the object it will start a timer and you must race back to the trophy location before the time runs out.
Unless you have played previous Batman games which explained this mechanic then this will be very hard for you to spot as the numbers on the map are very small.
Happy Riddle hunting!