A guide for the in-game secrets, represented by red question marks - some of them are easily visible, others are hidden. Collecting them all will unlock the "It's a secret to everyone..." achievement.
Chapter 1: Metro
Hidden in plain sight.
Chapter 2: Mansion
There's a lot of flowerpots, wouldn't you say? Let's break some!
Chapter 3: Apartments
I'm not 100% sure about this one, but it should be at the start of the level, partially hidden in the bushes next to the building.
Chapter 4: The Labs
You can see the secret in a small shed later in the level.
But how do we get to it? Well, what were we destroying this whole time?
Chapter 5: Warehouse
Oh look, those explosive crates again!
Chapter 6: Restaurants
Let's take a walk around the block ;)
When you enter the second part of the level, go all the way to the left.
Chapter 7: Slaughterhouse
Look at all that meat. Let's shoot it, just for fun!
Chapter 8: Junkyard
You can spot this secret among the stacks of wrecked cars.
Chapter 9: Hotel
You can see the secret locked in a small room on the second (third?) floor.
But who has the key, if not the bad guys?
Chapter 10: Police Station
Again, this secret can be seen in a small locked room on the second (third?) floor. When you grab the keycard, pick it up before continuing to the rooftop.