Secrets are everywhere in Doom: Eternal, and one notable secret that everyone will want to research is about the cute toys you can find in the game. The Hell on Earth level has three of them hidden in the game, and finding them is a little out of the way while you complete the game mission.
You can find the first toy shortly after starting the game on the houseboat. You can find it behind a hidden wall that you can smash to your left as you enter the room with the piles of undead crated in crates. The hidden wall is at the back, just before you find the block to access the level. Pierce the wall, follow the path and at the top you will find the toy.
The second toy can be found near the start of the second area, just before jumping into the partially destroyed building where a huge robot rests against a wall. Before you go down, turn left and you should see part of a floor that you can grab and go to the next level. Just around the corner, to the right, is the next toy.
The final toy at this level is about halfway through the mission. You can catch it after fighting a wave of demons near a store, the one with the blue sign that reads "Scott" above. When you're done fighting them, get on the semi-truck outside the store. To your left you should see a small lip of a cliff that you can jump on, above where a sign says 'Shuz'. Go inside, and further inside to the left, you should see a wall that you can knock down. Destroy it, go down to the bottom and the final toy will be waiting for you.
