Unsure where you need to go to find the Black Ops - Cold War evidence? The campaign is linked in more ways than you may realise, and this makes it even more fun to explore. It's quite easy to miss some of the items you need to complete side and optional missions, though.
First off, if you want to know which items to search for, you're accepting that you'll be encountering spoilers ahead. If you're happy to continue, then it's time to go over exactly what you need to find to allow you to ace the optional content in the campaign. Here's a list of all the Black Ops - Cold War evidence, the missions you can find it in, and the side missions that depend on it.
How to collect all the Black Ops - Cold War evidence
There are several missions in which you'll need to collect evidence that'll prove to be very valuable later down the line. Evidence can be an item that you pick up, or even a photo. My advice is to avoid rushing through the Redlight, Greenlight and Brick in the Wall missions, in particular. These missions contain lots of evidence, as well as optional tasks that you won't want to miss. Here are some missions to be aware of:
Take up photography in the Redlight, Greenlight mission
While Redlight, Greenlight is one of the main chapters in the Call of Duty Cold War campaign, it has an optional task that challenges us to collect six pieces of photo intel. Some of these items are tucked away in different rooms, so keep your eyes peeled and stay on the lookout for maps hanging on the walls. It'll take some roaming to find all six, but you can refer to my guide to take pictures of all the photo intel in the Call of Duty: Cold War Redlight, Greenlight mission first time around.
Unearth evidence in preparation for the Operation Chaos side mission
Before hopping into the Operation Chaos side mission, you need three pieces of evidence that you'll have to grab from other missions. The mission content is different for each player, so there are no shortcuts here, unfortunately. With regards to evidence, play the Nowhere Left to Run mission to find the Coded Message, pick up the Numbers Station Broadcast in the Brick in the Wall mission, followed by the Front page of the Observer newspaper in Redlight, Greenlight. Consult my guide on how to find the Black Ops Cold War floppy disk code and passphrase if you need some help.
Hone your skills of deduction in the Operation Red Circus side mission
Fancy weeding out which agents have infiltrated the European government? It sounds like you're ready to try your hand at analysing the Black Ops Cold War Operation Red Circus suspects. There are three pieces of evidence to uncover before you get started including Franz Kraus's Ledger from the Brick in the Wall mission, the Cassette Tape with Activities Report from Echoes of a Cold War, and an important Wristwatch containing the Dead Drop List from the Desperate Measures chapter.