Thanks for reaching us here. There is a good place for asking Power Virtual Agent related question: https://powerusers.microsoft.com/t5/Forums/ct-p/pva_forums
I have seen a very similar question to yours https://powerusers.microsoft.com/t5/Bot-Analytics/Check-the-question-that-is-not-recognised/m-p/813529
The answer for it is : "the unrecognized user utterance is automatically stored in a variable in the Fallback topic: UnrecognizedTriggerPhrase. While there is no in-build Analytics allowing you drill into the unrecognize user utterances in PVA, you can update the Fallback topic to save the UnrecognizedTriggerPhrase in somewhere at your side for further analysis." Just for your reference.
Hope you can go to that forum for more professional help.