According to this question there was a mechanism implemented to allow searching within sets of favorite questions, most usefully used when searching through your own favorite list (which for some users can be rather large):
According to the most recent comments on that same question, this feature got "broken" sometime in early 2011. Certainly, in trying to use this functionality myself, it does not appear to be working any more. I'm assuming this happened when the complete search engine was modified, as advertised in this blog post.
A simple enough request: Can we please have our favorite search back?
infavorites:is...no more, this would be terribly expensive on the back-end and just wasn't used enough to keep it around". I didn't have a long list of favorites myself, but I've heard of others who have found it useful.
status-deferred" answer at the top.