A room is patrol-free if no guards routinely patrol there. Whether or not a room is patrol-free is important strategic information:
In principle a room with only stationary guards can also be considered patrol-free, since it has these same benefits as long as the doors are not watched.
To find out whether a room is patrol-free, you can watch it from the outside by peeking through a door, watch it from cover inside the room, or watch through a hacked Security Camera or drone. A room can also be determined as patrol-free by listening for movement during the guard turn, though in this case you must take care to listen for the quieter sounds of some patrolling drones, and consider the shorter sound radius of Spec Ops walking in K&O facilities.
Watching or listening for one enemy turn is usually sufficient, because guard patrols normally have a two-turn cycle, meaning if they don't enter the room on one side of their patrol then they also won't enter on the other side of their patrol. However, guards can sometimes have longer patrol cycles (typically four turns) if their shortest path would intersect another guard's patrol path or a stationary guard's position. Even when this occurs, it usually doesn't cause the guard to be in some room on some enemy turns but not others, but in rare circumstances this can happen. Guards with the Reconnaissance Protocol trait, notably the Omni Harbinger, have a longer patrol route and can take around seven turns to complete a full patrol cycle. This makes early scouting in Omni particularly dangerous.
If you see that a door in the room is open, and there are no alerted guards in the facility, then the room is not patrol-free — this almost always means a drone patrols through that door.
Patrol-free rooms do not stay safe forever; after alarm level 5 all guards begin hunting in every room, so the most important way to keep a patrol-free room safe is to avoid unnecessary hits on the alarm tracker. It is also important to avoid distracting guards in nearby rooms, so that they do not enter the room or move through it while investigating the distraction.
Patrol-free rooms stop being definitely safe when a new guard enters the facility, until their patrol path is known. If possible, observe them as soon as they leave the guard entrance to see their interest point; if it is sufficiently far from the patrol-free room, then that room is still safe, otherwise it may not be. Note that if you distract a new guard, then their original interest point will no longer be visible by observing them, even after they have investigated the distraction.
Patrol-free rooms can also become unsafe when guard patrols change due to completing the objective in an Executive Terminals mission, or at alarm level 1 starting from the OMNI Foundry Lab mission in the extended campaign. This can be mitigated by distracting as many guards as possible before the patrol change is triggered, so that they do not take new patrol paths.
Since patrol-free rooms will not stay safe permanently, if you deploy a Portable Server or leave behind a dead or heavily-paralyzed guard, it is still worth leaving them in cover so that when guards do eventually enter the room, they are less likely to find something suspicious.