What is using mobile data while connected to wifi? Since a recent Android (6.01) update was pushed to my non-rooted Samsung G5 SM-G900I, while connected to wifi, the notification area shows quick 4G connection is made every three seconds (like a heart beat).

How can I find out what is responsible for this activity?
On Samsung Galaxy devices running KitKat and above, there is an option called Smart network switch, which if it enabled makes your device connected to a mobile network automatically when the Wifi connection is unstable.

You can disable it on Settings > Wifi> More > Smart network switch.