Since the XF developers are working on the big board search, I'll jump in with things I've built for our vB4 Sphinx search that we have ended up being fantastically useful for admins/mods...
Behind the scenes, we have 3 levels of searchable content...
- Content searchable by guests (just your standard public stuff... posts, etc.)
- Content searchable by logged in users (a user can for example use the normal global search and will get things that only THEY can search for... for example their private messages [conversations in XF lingo])
- Content searchable only by admins/mods (this includes things like users, user notes, etc.). We also make the contents of the changelog table searchable as part of a user search. So as an example, if you search for an email address, any user that EVER used that email will show up (even if it's not their current email). All parts of the user record are searchable within the "user search" for admins/mods... signatures, emails, custom profile fields, etc.
For mods/admins we can filter search by IP address as well... if you enter just an IP in the global search, it will return all content created by that IP. It's a filter type system so if you do a normal search query with an IP address in it, it will only return matching results that are related to that IP. So for example if you search for "billy bob 1.2.3.4", a regex sees the IP, strips it out of the actual query, and makes it find results that would come up for "billy bob", but then filter those results by that IP.
For user search, we've made it part of 2 different levels... some things about a user is searchable by anyone (public info basically... username, publicly viewable custom fields, signatures, etc.) and then mods/admins are able to search *anything* about the user.
Keep in mind that I built this to also be simple... it uses the normal global search, it does NOT push you into a complex sub-system where you can search a specific field (you can use the built-in user search in AdminCP for that)... It's quite handy for tracking down spammers for example... pop in the domain into the global search and you not only see normal content, you can see users that might have used an email within that domain historically or users that have a signature pointing to the domain.
Mods/admins have come to rely on it so much, I suspect I'll end up needing to extend XF's big board search to include similar functionality if it doesn't do things like that out of the box.