There's a whole bunch of Mysterious Objects hidden around Union - and they look awfully familiar.
The Evil Within 2 has a number of hidden Easter eggs stashed throughout its chapters, all of which are references to other Bethesda games; Doom, Prey, Quake, The Elder Scrolls, Dishonored, Wolfenstein, Fallout and the original The Evil Within.
Once you find these they are displayed in Sebastian's office. If you're looking for more useful items, you're going to want to find all the weapons including the magnum and shotguns, track down the Warden Crossbow and also find and fix the broken sniper rifle. When you have all this, make sure you're using the safe houses regularly.
Got all those? Good. Let's get some nice little toys for your trophy room.
Dishonored - Corvo's mask
You can find this during your explorations in Chapter 3. It's in the abandoned train but look out for enemies and traps nearby. You want the train carriage with body parts on the floor and lots of rats running around. It's on seat at the far end of the carriage.
Wolfenstein - Panzerhund
You should be checking inside trucks anyway, because you'll almost always find something of interest in it. The robot dog thing from Wolfenstein is inside the back of a truck outside Treadwell Trucking, which you can get to during Chapter 3. Smash a crate to find it.
Quake - Rocket Launcher
Unfortunately it's only a replica, but you can find the rocket launcher from Quake on the roof of a house in the Residential District, again during Chapter 3. You have to climb up there via a shed (you'll see boards from the shed to the house) which itself is inside a chain link fence. All of this is near to the House Inn.
Fallout - bobblehead
You can pick this up during Chapter 6 - On The Hunt in The Marrow: Facilities, but it involves backtracking. The second time you meet The Hunter and escape from its grasp, go back into the storage room and you can find the Fallout bobblehead in the corner between two stacks of crates.
Doom - action figure
Those action figures you found lying around in Doom also happen to be in The Evil Within 2. During Chapter 7 you can find this Doom figure in the Union Business District. It's near the corpse of a Mobius operative, inside a large bin just around the corner from her body.
The Elder Scrolls - trophy
This is the easiest to come across. As you wake up in Chapter 12, check out the upstairs office in the house and you'll see this Elder Scrolls trophy on the desk.
Prey - mug
Another easy one to find, this is behind the reception desk of the Sanctuary Hotel during Chapter 13.
The Evil Within - The Keeper bobblehead
And finally to top it all, there's also a bobblehead of your old buddy The Keeper from the original The Evil Within. This is on the shelf in the living room of Sebastian's house hiding in plain sight.