the Orange Circle with Question Mark is indicative of the fact that code model inside FB is unable to recognize or locate the definition for the identifier used.
Sometimes the code model can get out of sync with the code and start indicating this incorrectly. Though many workflows leading to this condition have been fixed there are still chances of this happening.
Normally when this happens other features like code hinting, ide productivity features like quick assist also stop working for that particular identifier.
Though not ideal, you can try closing and reopening the file/project and see if this goes away. If the problem is very persistent please log a bug with a sample project so that we can fix this.
You can turn off this feature (without any side effects) by going to "Preferences->FlashBuilder->General->Report problems as you type".