This is using Sitecore 8.1 Update 1, Coveo on-premises version 7 (8388.7) free edition, and Coveo for Sitecore version 18.104.22.168.
I have a document that's been secured such that everyone is denied access to it in inheritance, but then certain groups are granted read permissions, so that only those in the specified groups can see the content. Looking at the Effective Combined Permissions list for my item in the index browser, if I search for my account I see that it is marked as Allowed, so I'd expect to see the content in my search results. However, when I perform a search, the content isn't visible in the results.
The only advanced expression I have in the query is that it needs to have a layout, be part of a particular site, or it needs to be in a particular path. In this case, the path qualifier comes in as well as having a layout. So nothing is immediately jumping out as to why my content wouldn't appear on the site. I can browse to the content just fine. In my Sitecore logs, I do find the following error multiple times, for user accounts I know to exist...this is a good example. I'm not sure if this error could be a reason the security isn't being honored correctly.
10804 00:00:51 ERROR An error occurred while calling method "GetUserMembers". Exception: Coveo.Connectors.Sitecore2.SitecoreWebServiceExceptions.SitecoreWebServiceUserNotFoundException Message: The user "xyz\anonymous" does not exist. Source: Coveo.Connectors.Sitecore2.SitecoreWebService at Coveo.Connectors.Sitecore2.SitecoreWebService.Wrapper.BaseSitecoreWrapper.GetUserMembers(String p_UserName) at Coveo.Connectors.Sitecore2.SitecoreWebService.SitecoreWebService.<>c__DisplayClass1d.<GetUserMembers>b__1c() at Coveo.Connectors.Sitecore2.SitecoreWebService.SitecoreWebService.TryCatchWrapper[T](Func`1 p_Action, String p_MethodName)
The Coveo diagnostic page shows all green, except for the Coveo Search Web Service, which presents this error. I don't know if there's a possible correlation here.