Saul, even if you are not actively using your computer or other devices, they can use data. Phones will use data if they are connected to your home wifi, regardless of how much cellular data your plan has. Background app refresh on phones, automatic updates on computers, windows 10 sharing updates you have via the internet and, more all use data when you aren't even using the devices.
You can email your account and contact information to email@example.com and, request a usage review to get a better ide of where your data is going but, they cannot tell you exactly, 100% what device used it.
For example, I have 2 Windows 10 computers and 2 iPhones but, Exede sees only one of each connected to my wifi. So that can tell you if it was an Android, iPhone or Windows phone but, not the phone number of the device or, which one if you have multiple devices that are the same operating system.