Install this app:
https://play.google.com/store/apps/details?id=com.antispycell.connmonitor
That has a monitoring tool which will allow you to monitor which apps are sending/receiving data and where it's going/coming from.
Happy new year to you too!
I just installed this app, and yes, it does report which apps are sending data out.
However, it does not resolve the OP's question as to what, if anything, is using his
MobileData connection. As a matter of fact,
it does not report How Much data is being sent for each app. Only that it is connected and to where it is connected.
IF you manually turn off both radios, and then restart the Network monitoring tool, you should be able to verify IF there is any traffic at all.
My personal method of controlling MobileData is to use MacroDroid to turn off the WiFi and Data radios when the screen is locked. My signature shows that.
So far, I have not run across an app that will tell you if, or how much, the MobileData channel is being used. I have always assumed that if the MobileData radio is turned OFF, then it cannot send/receive any traffic. That so far, has been a fairly accurate assumption because my monthly MobileData usage is less than 250 mBytes/month. and that is with 3 cellphones all sharing the same DataPlan.
both my wife's and my S5 phones both have MacroDroid controlling the radios. My daughter's phone automatically switches to WiFi if one is registered and close by. She is very cautious to never use Data away from home unless it is necessary.
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Verizon Galaxy S5 since Launch Day, using ART
Box Stock, not rooted
Nova Prime, Textra, Blue Mail
MacroDroid senses Scrn off, turns Wifi & Data OFF,
MD senses Scrn on, turns WiFi & Data ON
