A type of marker to indicate that players should not proceed unless a member of staff is guiding them. From the Accelerant core rules:
"Some games have locations in a game site are normally accessible to players, but are sometimes closed for specific encounters or modules. While this is usually handled with yellow information signs, it is often difficult to spot and read information signs without entering the encounter area to read them, especially at night. To make this easier, areas can be marked with circular information signs (rather than the usual octagonal signs) and at night these signs are lit with circular rings created with looped glow bracelets. If you spot these Turn Back Markers hanging in the area, it means that the area is inaccessible and you should stay away unless you are led to the area by a spirit, a scout, or a similar guide."
Just because an area has a Turn Back Marker does not necessarily mean the marker will stay there for the duration of the event, unlike an Out of Game Area. Players may return to the same location later to find no markers at all barring their passage.
Because these areas can change rapidly, and vary based on what events are taking place, the in character reason a player uses to avoid the area is left to the individual player to role play as they see fit. This differs from an Out of Game Area, which clearly state what the obstruction is. From the Accelerant core rules:
"It is often left to the player to explain why they cannot proceed when they spot Turn Back Markers. One common reason is that the area ahead is impassable swamp. Another is that a mist has risen and players cannot find their way through the area. Sometimes players simply state that the area ahead looks uninteresting and turn back. It is left to you and your companions to come up with a plausible reason that you cannot proceed when you spot a Turn Back Marker."