Hidden bonuses: 6
Level walkthrough: Exit the cavern and keep going straight. Ignore the comment, because Spencer will be far from safe here. Turn left once you've reached a small junction and eventually you'll get to the first bonus item. Continue moving forward to find a pod with a sniper rifle. You can start using this weapon right away or you can wait for Spencer to get closer to his enemies.
I would strongly recommend that you stay close to the river where you've collected the rifle. Head forward and inspect the area to your right. Punch several rocks with your bionic fist (Y button) to uncover the second bonus item. Take the object and start moving towards a nearby building.
It would be good idea to reach one of the balconies of the building as soon as possible and this can be done if you use the cliff to the left. Eliminate a new sniper upon your arrival and prevent Spencer from being attacked by several snipers at the same time. Hide behind pillars and other large objects if you're in need of healing.
Make sure that you've killed all enemy snipers before moving on! Continue climbing to the top of the building. Once you're there look around to find the third bonus item. Notice that you can attack new enemy soldiers from here. Eliminate all of them, drop down and hack into the relay station.
Approach the ledge carefully and you should notice a new minefield in front of you. Naturally you must start using the mines to get to your destination, however you'll also have a chance to collect the fourth bonus item along the way. Eventually you'll end up inside a new cavern. Collect the ammunition and proceed to the exit.
Keep running towards your current destination. Thankfully in this case you won't be bothered by any difficult fights. There are enemy guards stationed near a new relay station and obviously you'll have to get rid of them. Hack into the station once they're gone to disable the mines.
I would recommend that you now spend some trying to reach two remaining bonus objects. In order to find the fifth bonus item you must get to the top of a nearby cliff. The object is located in plain sight, so you won't have to spend too much time searching for it.
The sixth bonus item is more difficult to find. You would have to backtrack a little bit to locate an unexplored cliff to your right. Make sure to avoid irradiated zones while trying to find the collectible. The object you're looking for is located near the abyss and from there you can also see the building you've explored at the beginning of this level.
Return to the relay station once you're done exploring. Go to your right and you should notice a new minefield in front of you. Make sure to be very precise, because all the mines are far away from each other, so it'll be easy to make a mistake. Get to a new ledge and head to the exit.