On this page of the Back 4 Blood guide, you will learn how to unlock door with a yellow logo and what you can find behind it.
What do they look like?
Locked doors appear in almost all maps when it in campaigns. They are often marked with a yellow logo that resembles a rifle scope, but they are always closed.
How to unlock them?
To open a door with a yellow logo, you need to have a certain item, namely a Tool Kit. A set of tools can be found in some boxes, however, this is a fairly rare occurence. It's best to buy the Tool Kit in the store, which is located in the safe room. Once you have this item in your possession, go to the door and use the Tool Kit.
What can you get?
Unlocking doors often rewards you with items that make sacrificing the Kit worthwhile. You can find CP, which is the currency of the game, improvements to weapons, ammunition, meds or even high-end weapons. It's definitely worth owning at least one tool kit. This is not a big expense, and it definitely pays off to unlock them doors.