Scattered across each map are 30 collectible items. These aren’t marked on your map, but are rather easy to find. Some helpful tips:
-Many collectibles can be found on rooftops
-Use your jetpack. Collectibles can be found much easier when searching from the skies.
-Level the town. When you can’t find any more collectibles floating around on rooftops, it’s time to destroy the buildings. Some collectibles are hidden in alleys between buildings or even small spaces in the building you wouldn’t usually explore.
Detailed locations can be found in one of two collectibles guides:
Complete Collection Guide