As others have said your smart phone apps will use data in the background. Especially if you have background app refresh and automatic updates of them turned on. This can be disbabled on Apple devices Settings>General>Background app refresh>OFF but im not sure on windows and android if this is an option im afraid.
Another thing to consider is any TV boxes you have connected. Having a sky HD box connected for example will use data and also syncing emails that contain attachments all contain data. But yes I agree 25gb is a considerable amount in 6 days.
Presuming your network is secured with a passcode, are you certain someone hasnt got hold of your network key and is using your connection? A friend had this happen recently when he had neighbours round socially and shared his key, once their device remembered the key everytime it came within range it connected to his router. A simple way of checking this is to visually check whats connected to the router in the settings page. Theres an old thread i've found below which tells you how to see;
https://community.bt.com/t5/BT-Infinity-Speed-Connection/Who-is-connected-to-my-router-internet/td-p...
Just some food for thought and hope you get sorted
Jamie
