Extract /var/mobile/containers/data/application from an IPhone or backup
Apps can choose to not include data in their container from an iTunes backup, so the folder won't appear in a backup. Snapchat do this, which means the container isn't backed up when you do an iTunes backup and the data inside won't be able to be accessed.

You can't access other app's containers without jailbreaking. Unfortunately, this means that data isn't accessible.