Excellent. Now hopefully you are getting acquainted with maze of 'Settings'.
In case your cellular bandwidth is continuing to be tapped by ghost apps and you are not on 'the spookes' radar and bugged by them then there are a few relatively low demand services worth examining.
Settings > Notifications (Phone) > turn slider OFF
Settings > Backup and rest (Personal) > Back up my data slider OFF
(I never trust clouds for personal info storage...sorry off topic)
Settings > Location > slider ON
Settings > Location > Locating method > GPS only
Settings > Location > Improve accuracy > all sliders OFF
If after all this you still have a leak then it may be time for factory reset, but let us wait and review what you find.
Add to the above - Verizon Wireless has embraced data mining in a big way. This done by profiling cell phone plan usage by customers. They have also been gracious enough to share individual profile with their clients. You will find this in their App 'My Verizon' > Data Hub > Details > Line Details.
This shows the categories of app and the data bandwith usage.
Should help you monitor what is happening. Hope all this helps you sort out your mystery. Incidentally, FB and Twitter are notorious in background data (cellular) use. Cannot help with these two as they are banned on my Cellular devices (by me).
I've found the data use is with particular apps. The easiest I've found for testing is the Facebook Messenger (while video chatting)
I've also found that if I begin using the apps (say for video chat) then turn off mobile data, the app functions as it should over Wi-Fi without any issued. I can then turn Mobile Data back on and the applications will continue to use Wi-Fi. However if I do not turn off the mobile data the applications will continue to use mobile data versus the available Wi-Fi. Either the applications are ignoring the phone settings preferring mobile data, the applications are ignoring the availability of Wi-Fi, or the problem could be with VZW settings not telling the applications to use the availble Wi-Fi.
I would flat out blame Facebook application creators for poorly written software but I've found other software with similar behavior.
Thank you for helping with this issue.
Kudos to you NitroNeo for your 'stick-with-it-ness' and getting down to nitty-gritty. No doubt this will alert and benefit all who are interested and seek an answer.
You should definitely share your observations with FB designers. They are a very good bunch and may have made a conscious decision to favor cellular over wi-fi at start-up as opposed to evaluating user preferred parameters or doing a least-cost alternative, each time. These additional tests add to the overhead-size of the app.
Way to go and thanks again for sharing.
In addition to this chain... I can now claim that these problems are not being seen in the Google Pixel XL so this could have been strictly a Samsung problem as I can also verify that a similar problem exists with the Note 5.