On my last project, we had to deploy SharePoint within a specific AD forest while users and groups were located within another AD forests.
We have implemented a One Way Trust between the SharePoint AD Forest and the users/groups forests; everything was going fine except for the search results.
Indeed, all search performed were returning no results while we knew we should have results.
After some analysis, we discovered user accounts within the SharePoint forest were able to get results while users from the trusted forests not.
We finally found a Microsoft KB article which explains the reason and provide solution (KB 2344518 – http://support.microsoft.com/kb/2344518)
According to this KB, this due to how the Search Service Application is performing security trimming.
The solution was to force the search service application to store ACL’s in Claims format (even if we were not using Claims authentication for web applications):
$SearchApp = Get-SPServiceApplication