Should a retrieval method return 'null' or throw an exception when it can't produce the return value?

If you are always expecting to find a value then throw the exception if it is missing. The exception would mean that there was a problem.

If the value can be missing or present and both are valid for the application logic then return a null.

More important: What do you do other places in the code? Consistency is important.

Only throw an exception if it is truly an error. If it is expected behavior for the object to not exist, return the null.

Otherwise it is a matter of preference.

As a general rule, if the method should always return an object, then go with the exception. If you anticipate the occasional null and want to handle it in a certain way, go with the null.

Whatever you do, I highly advise against the third option: Returning a string that says "WTF".