Site Collection not returning search results

Let's say you are doing a "Search This Site" within your site collection/site and you are getting nada but when you click on "Expand this search", you are getting results, including from the site you were looking for.

So, your content is being crawled, no silly top level errors etc. So what is causing this across your site collections but your global search is working?

THIS

You need to ensure that whatever is set as your default zone in alternate access mappings IS the same as the URL that you are crawling.

So, in Central Admin, go to Configure Alternate Access mappings and check your Default URLs

Then, go into your Service Centre App and check the content sources, the url should map. There shouldn't be a difference in http/https or anything, they need to be identical.

Run an incremental

Wow! Look at that, bloody fixed!!!!

Note: This was caused by me messing in the first place and then not remembering what I messed with!!!! Be careful in SharePoint land.