I recently encountered peculiar problem when configuring FAST for SharePoint server in my lab environment. I was using the MS technet document
After configuring the two Search Service Apps (Content and Query) and starting full crawl, I was able to see items in the default search content collection "sp" by running
Get-FASTSearchContentCollection -name "sp"
The problem was, even though the items were supposedly in the index, I was unable to get any results while searching. This was true for both searching via SharePoint FAST search center site, and when trying to use the built-in QRServer on FAST (which can be accessed locally on the FAST server by browsing to http://localhost:13280 if you keep all defaults during configuration, or at http://localhost:base port+280 if you changed the base port)
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To make things even more interesting, I was able to feed a document manually using:
Docpush -c sp <filename>
and this document was showing up on both QRServer and SharePoint FAST search center with no problems.
The problem turns out, is I had added the fast service account to the local administrators group on the FAST server, IN ADDITION to it being in the FastSearchAdministrators local group that’s created by default after FAST configuration.
This I assume caused search results to be security-trimmed in very unexpected ways.
The fix was easy enough: remove the fast service account from local admins group on FAST server, reset the index on SharePoint, on the Content SSA, followed by cleaning up the sp content collection on FAST.
After re-indexing on the content SSA, documents started showing up in search results with no problem.