Somewhere/Anywhere

Hi,
All grammars say that somewhere should be used in affirmative sentences and anywhere in negative or interrogative sentences, but, as a non-native English teacher I struggle to find a good explanation for exceptions such as: "Did you go somewhere interesting on Sunday?" Most grammars say we use somewhere in questions when an affirmative answer is expected, but this is not an explanation that foreign students, or myself, understand. As one could answer this question either positively or negatively.
The best explanation I have found is that you use "Did you go somewhere interesting on Sunday?" when you know that the person you are addressing your question to DID go out, and that "Did you go anywhere interesting on Sunday?" would be also correct, except this would mean you do not know at all whether the person went out or not.
Do you agree with this? Any better/clearer ideas on how to explain this?
Thank you