The open levels in Marvel's Avengers will give you a number of opportunities outside of the main quest - there are AIM prisoners to save, safes to open, and loot huts to break into. These loot depots aren't exactly the hardest puzzles out there, but you might not be sure how to open those doors at first. Once you get the hang of it, you'll be looting without a hitch from now on.
Floor panels
You will likely see your first AIM repository in the campaign's Missing Links mission. Some of these bunkers will use switches on the ground that glow yellow. Climb on one of them to make it blue; there will usually be more than one floor switch, so pass them all, and once you do, the switches will turn green and the door to the AIM depot will open. If you can't find the panels, use tactical awareness by pressing the D-pad; you will see cables connecting the door to the panels.
Switches (melee attacks)
Near a locked AIM bunker door, you can find these odd yellow hexagonal stations. These are the switches to open that door, and launching attacks until they turn blue will activate them. There are usually four per door, and Tactical Awareness can help you find exactly where they are. Outside, you can see one on either side of the door, then two more above them on the second floor. Inside, these switches are scattered around - walk around the door to its left to find a switch on your right. Another is behind the loot room. The other two should be on the second floor above the door. After hitting all four, the blue turns into friendly green.
Switches (remote attacks)
Some switches to open doors require ranged attacks instead. In some cases, you may find two switches, one on each side just above the loot depot door. But in the large open levels, you will usually find these round switches on the towers adjacent to the depot. Sometimes these towers will only have one switch to touch. But for the most part, these towers will be on the left of the depot and will have four of these switches. Hit them from the bottom up - one in the front, one in the back first, then look in the middle section of the tower (they're divided into thirds by platforms) for a switch on the right to hit that. The last switch will be on the top of the tower on the left side. The switches will turn from yellow to blue to green, and there will be a nice little chime when they turn green and open the door.
Terminals
The easiest way to open some of these doors is through hackable terminals. Just go to the terminal, follow the button prompt (square on PlayStation or X on Xbox) and the door will open. However, players have found that only Iron Man and Black Widow are able to open these doors - likely because they are the most tech-savvy Avengers. Other characters will just get an “ACCESS DENIED” message, which can be frustrating if you're playing as a different character in single player.
Murs cassables
These big broken doors just ask to be opened. As the Hulk, Ms. Marvel, or Thor, hold down the Heavy Attack button and you'll unleash a powerful movement that will smash the door. Unfortunately, Iron Man and Black Widow have no native way to open these doors. Iron Man has a workaround for breaking open walls using the Hulkbuster, but some would rather retain this ability for combat instead.