Click on the red door to display the labels of the rooms it connects. Then, click on the nodes of those rooms to verify whether they should be connected. If the issue is not related to polygons (see below), the red door can be safely ignored. Otherwise, edit the room polygon to resolve the issue.
This occurs when two corridor rooms share overlapping polygons. You can usually identify this by the unusually high density or close proximity of nodes in those rooms.
After modifying any room polygon, remember to run Detect Walkway for each updated room!
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