The error list in Visual Studio is by default only showing opened files even though the filter is set to Entire Solution.
This is due to a default settings in Visual Studio. To change this, navigate to Tools -> Options -> Text Editor -> C# (or other language) -> Advanced -> Show compiler errors and warnings for:
Here you have the option of None, Current document, open documents and entire solution. The default is set to Open documents. Changing this will make Visual Studio analysis your codebase more so please be advised.