The latest IDE has changed the UTI names for the old project extensions once again, making the previous version of the Spotlight Importer not import files ending in .rbp any more. The new version 3.5 fixes that:
http://www.tempel.org/RB/SpotlightImporter
If you’re using a Mac for development, I recommend installing this importer as it makes it easy to use Spotlight to find anything in your projects.
See also my article on various ways to search your projects (all platforms): http://blog.tempel.org/2015/10/SearchingMultipleXojoProjects.html