I first saw one a few months ago, above the side entrance of Union Jacks club on East Burnside Street, and then another outside the Kingston pub on West Burnside.
The signs indicate a building that Portland Fire & Rescue has deemed "unsafe." From a PF&R memo from December: "The Unsafe Buildings Alert Program provides a tool for identifying structures that, due to their construction, use, or state of maintenance, pose extraordinary hazards to emergency personnel."
There are currently signs on a couple dozen buildings around town. If one of the buildings catches on fire, PF&R crews will attempt to fight the fire from outside rather than enter the building. They will go inside if they need to perform a rescue, for example, but will keep their time inside the "unsafe" structure as short as possible.
Any of these dilapidated death-traps in your neck of the woods?