As you see you can now use RB Class in the search criteria popup and look explicit for projects using a given class name:
On April 5th, 2016, Thomas Tempelmann has released a new version of the Spotlight importer for Xojo.
The latest Xojo 2016r1 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:
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 Thomas' article on various ways to search your projects (all platforms): blog.tempel.org/2015/10/SearchingMultipleXojoProjects.html
And of course you have full text search in your Xojo and Real Studio projects.