Re: My Samsung Galaxy 5 is using mobile data when not connected
Welcome to the community and thanks for posting.
Koala5 is absolutely correct here. Whenever WiFi is turned off the phone will automatically reconnect to the mobile data (if this is also on). If it's left on then apps in the background will use the data.
To turn mobile data on and off just run your finger down the screen from top to bottom to reveal the notification panel. Find mobile data and switch it on and off when you want.
Hope this helps. If you need further help just ask.