Hidden bonuses: 4
Level walkthrough: Exit the tunnel and proceed to your left. It shouldn't take you too long to find the first bonus item. Grab the nearest tree branch and swing a few times to add it to your list of collectibles. You may now begin moving towards your current destination.
Make sure to choose the main path as you're getting closer to the clearing, because you'll be dealing with large doses of radiations here. Watch a new cut-scene and explore the surrounding area once it's ended. Collect the second bonus item from the ground.
Approach the cliff and make a jump towards a large structure seen in the distance. Grab the ledge and get on top of this structure, because that's how you'll find the third bonus item. Drop down and head forward.
As you've probably suspected, you'll have to get inside a building located nearby. It shouldn't be a problem, especially since you won't be stopped by anyone. Once inside take your time to find the last fourth bonus item. Afterwards proceed towards the large exit.