There are 22 Hidden items in the game, three in each mission (except mission 1 and 9 with zero and mission 2 with 4 items). As always, some of the Hidden items are well hidden, and others are placed in easy spots. The Hidden items are always different things, sometimes a coin, a deck of cards, a pennant, a gas mask or the propaganda posters to unlock the propaganda missions.
In the spoilers of each mission, you will find the map of the mission and the marked Hidden items (orange dot / point), so they should be easy to find. If you have any problems or need help, videos for each mission are also available.
Maps:
I created these maps for myself to find all the collectibles and determine a good path / order for my videos but I think they are good enough to help you here as well. I might create new maps as soon as I have more time. (No guarantee). The documents are marked with a orange dot / point.
Mission 2 (Dead Drop)
Mission 3 (Sonderzüge Sabotage)
Mission 4 (Collision Course)
Mission 5 (Devil's Cauldron)
Mission 6 (Assault on Fort Rouge)
Mission 7 (Lock, Stock and Barrels)
Mission 8 (End of the Line)
Videos:
The videos are complete guides for the individual missions, so they show all the collectibles and not just the Hidden items. In the spoiler of the missions you have the video, the timeline of the Hidden items and an overview of the Hidden items in the game, so you can find missed Hidden items very quickly. The numbers behind the Hidden items in the timeline match the sorting in the game.
Mission 2 (Dead Drop)*** Spoiler - click to reveal ***00:39 - 01 - Hidden Item #4
05:43 - 13-16 - Letter #3, Hidden Item #1-#3
Hidden Items:
#1 - Police report: #222
#2 - Police report: #223
#3 - Police report: #224
#4 - La Resistance
Mission 3 (Sonderzüge Sabotage)*** Spoiler - click to reveal ***01:01 - 03-05 - Hidden Item #3, #1, Letter #3
02:14 - 06 - Hidden Item #2
Hidden Items:
#1 - Resistance Flag
#2 - Gestapo ID Badge
#3 - Le Sniper
Mission 4 (Collision Course)*** Spoiler - click to reveal ***02:02 - 06-07 - Document #1, Hidden Item #1
07:31 - 20 - Hidden Item #3
08:10 - 21-22 - Document #3, Hidden Item #2
Hidden Items:
#1 - Wine Bottle
#2 - Gas Mask
#3 - Liberation
Mission 5 (Devil's Cauldron)*** Spoiler - click to reveal ***00:39 - 02-04 - Rifle Workbench, Hidden Item #3, Letter #1
09:24 - 15 - Hidden Item #1
09:49 - 16 - Hidden Item #2
Hidden Items:
#1 - Playing Cards
#2 - Laboratory ID
#3 - Le Maquis Voit Tout!
Mission 6 (Assault on Fort Rouge)*** Spoiler - click to reveal ***01:10 - 04 - Hidden Item #1
03:03 - 07 - Hidden Item #3
12:54 - 20-22 - Rifle Workbench, Document #5, Hidden Item #2
Hidden Items:
#1 - Todt Uniform Badge
#2 - Committee C Map
#3 - Prende Le Maquis
Mission 7 (Lock, Stock and Barrels)*** Spoiler - click to reveal ***00:23 - 02-03 - Hidden Item #3, Rifle Workbench
04:08 - 10 - Hidden Item #2
07:34 - 15-17 - Hidden Item #1, Starting Location (Western Farmhouse), SMG Workbench
Hidden Items:
#1 - Engraved Lighter
#2 - Gold Pocket Watch
#3 - La Voix Du Maquis
Mission 8 (End of the Line)*** Spoiler - click to reveal ***00:35 - 02-03 - Hidden Item #2, #3
01:26 - 04-05 - Stone Eagle, Hidden Item #1
Hidden Items:
#1 - AA Repair Manual
#2 - New Tank Blueprints
#3 - Pour Une France Libre
If you have any questions, just ask. And just in case you like to have everything in one video, here is a video that includes all collectibles. (added also below Target Führer DLC)
All in OneLights, Camera, Achtung - Target Führer DLC