Notepad++ is not registered with the USPTO (U.S. Patent and Trademark Office). Searches around will turn up nothing in the U.S. database. The name is trademarked in France (via INPI, the French patent office), which is why the maintainer has been able to send cease-and-desist notices in recent cases (e.g., the unauthorized “Notepad++ for Mac” site).

In the U.S. its only covered by common-law trademark rights from long use, as there’s no federal registration.