Are empty interfaces code smell?

You state that your function "returns entirely different objects based on certain cases" - but just how different are they? Could one be a stream writer, another a UI class, another a data object? No ... I doubt it!

Your objects might not have any common methods or properties, however, they are probably alike in their role or usage. In that case, a marker interface seems entirely appropriate.

If not used as a marker interface, I would say that yes, this is a code smell.

An interface defines a contract that the implementer adheres to - if you have empty interfaces that you don't use reflection over (as one does with marker interfaces), then you might as well use Object as the (already existing) base type.

Although it seems there exists a design pattern (a lot have mentioned "marker interface" now) for that use case, i believe that the usage of such a practice is an indication of a code smell (most of the time at least).

As @V4Vendetta posted, there is a static analysis rule that targets this: http://msdn.microsoft.com/en-us/library/ms182128(v=VS.100).aspx

If your design includes empty interfaces that types are expected to implement, you are probably using an interface as a marker or a way to identify a group of types. If this identification will occur at run time, the correct way to accomplish this is to use a custom attribute. Use the presence or absence of the attribute, or the properties of the attribute, to identify the target types. If the identification must occur at compile time, then it is acceptable to use an empty interface.

This is the quoted MSDN recommendation:

Remove the interface or add members to it. If the empty interface is being used to label a set of types, replace the interface with a custom attribute.

This also reflects the Critique section of the already posted wikipedia link.

A major problem with marker interfaces is that an interface defines a contract for implementing classes, and that contract is inherited by all subclasses. This means that you cannot "unimplement" a marker. In the example given, if you create a subclass that you do not want to serialize (perhaps because it depends on transient state), you must resort to explicitly throwing NotSerializableException (per ObjectOutputStream docs).