When you get to Level 20, you can buy the ability to find all collectibles.
Remember to choose your GPS's legend to collectibles so u can mark a route to the collectible on the GPS.
Some of those collectibles are on the rooftop or in a cave that hard to discover.
Make sure u have a chopper or later in the game a VTOL to do this and makes it easier & faster.
If you find that you stand on the collectibles logo but nothing there, find the nearest river and search by the shore, those are inside a cave underground but if you look carefully, that should be okay.