Why does CSharpExtensions.ClassifyConversion return an explicit reference conversion for object / string?

Question

The CSharpExtensions.ClassifyConversion(Compilation, ITypeSymbol, ITypeSymbol) method returns an explicit reference conversion if called with the special type object as source and the special type string as destination argument:

var objectTypeSymbol = context.Compilation.GetSpecialType(SpecialType.System_Object);
var stringTypeSymbol = context.Compilation.GetSpecialType(SpecialType.System_String);
var conversion = context.Compilation.ClassifyConversion(objectTypeSymbol, stringTypeSymbol);

As far as I know C# doesn't specify a built in object to string conversion: C# language reference. Why does it return an explicit reference conversion and what does it imply? How would the c# syntax of such a conversion look like? If ClassifyConversion is called with an empty user defined class as source argument and the string special type as destination argument, NoConversion is returned as expected.