If you are running Xamarin projects in Visual Studio 2015 you maybe have a lot of error related to assemblies that not is referenced.
Even if there are errors the project will build successfully. Microsoft says this is a Xamarin bug and that the have reported it to Xamarin.
Until it's fixed there are a workaround. On the top of the error list there are a filter where you can select source for the error list. The default source is Build + IntelliSense, if you select to just show errors from build the errors will be gone from the list.