Not sure if this is a documented feature but you can search within Google+ Communities by adding s/yoursearchterm to the community home address (operators like AND/OR also appear to work).
I have some students using a private community for group work, and they have to provide a list of posts for assessment.
The only way we’d found to search just that community was to scroll down the page to the start of the community -then use the browser’s search feature, but that only found the posts they’d started, not the discussions. I have just tested this with my most prolific student & seem to have found both his posts & his comments.
I tried the ‘s’ switch in the url after seeing a Googler share this link which lists all the ‘Apps Script’ communities https://plus.google.com/u/0/s/”Apps Script”/communities so it looks like it can be used elsewhere in the url.
I notice however that using this didn’t always return the expect result, which is worth bearing in mind.