That's why it pays to register your copyrights with the Library of Congress. Anyone can declare material that is not copyrighted as their own.

I recall when I worked for the then Bell System, the old round blue and white belll logo metal signs that were affixed to a building advertising that a public payphone was inside ... was not trademarked. Somebody took the time to trademark that logo on the public phone sign and sued Ma Bell... who paid, and paid, and paid.