"When the DTR app receives a QuestionnaireResponse but does not receive a Questionnaire URL, it should check the QuestionnaireResponse for the context extension"

A QuestionnaireResponse will always have either a QR.questionnaire or a contained Questionnaire with a 'derivedFrom' that points to the canonical.  The 'context' is useful for searching to figure out what order or coverage is relevant, but there's never a situation where a QR won't know its Q.