During infiltrations, you’ll be hacking more than your fair share of security doors. Some doors are unlocked and can be pushed open. Some security doors are locked but are quickly hackable (colored light blue). However, some security doors are locked with an access key. They have the access key icon next to them; if you hack a locked door like this, it informs you that an access key is needed.
The key will be in one of two types of locations: on an enemy guard or on a computer (usually a laptop). Obtain the access key by cycling through security cameras until you can spot your access key–carrying target, or use the Quadcopter, RC Jumper, or Marcus with a clear line of sight.
As soon as you, a drone, or a security camera hacks an access key, it becomes available to Marcus. You keep the access key for the remainder of the specific operation.