I'm not sure this is the case. I think it could just as easily be "our guard". Also, Duo routinely delivers sentences with "suo" that lack an antecedent, so I don't think this rule can help users know how to answer the question.
I think you are right. Quite possibly without context there is no determinate sense - 'our/youjr/his/my guard. Is Non è il momento di abbassare la mia guardia ( .... to lower my guard) correct. I find that you can get stuck on things like this in Duo!