Welcome to the Community Forums! We appreciate you coming here for help with your mobile data query. Sorry for our delayed response.
If you find that your data allowance has been used when you are certain that you have been using WiFi, I can only advise that your device must have dropped the WiFi connection and back ground apps have used a small amount of data e.g app notifications.
Please let me know if this is still a concern for you - if you sign into your My Virgin Mobile account, and view your monthly bills, a breakdown of the data usage will be listed with the times it was used and how much was used ect.