What’s up with the “U” signs on some Portland buildings?

Have you noticed these signs on buildings throughout Portland?

What are they? Our best guess was secret lairs for Uber Drivers.

The signs are actually put up by the Portland Fire Marshal to mark properties that may be unsafe for firefighters. A building gets a U sign if a fire inspector and city engineer determine together that it does not meet fire codes.

This does not mean the buildings are unsafe to inhabit. A building can be up to building code but not comply with the fire code for various reasons.