Another part of the DexNav is the ability for you to scan Pokémon you see in the overworld. When you are close to the Pokémon, the DexNav will light up blue to indicate there is a Pokémon nearby. Often these Pokémon are on the ground, but often they will be flying or swimming by. If the DexNav circle is pressed, then the Pokémon will be scanned, it'll be added to your search level if it's the first time you have encountered it, and it will be scanned in your Pokédex.
Most of the Pokémon can be found in a variety of areas, but there are a couple that are quite rare and more easily seen through this method than any other, such as Lucario.