Who I have the same questioned this topic

For the last several months, data has been draining out of my wife's phone while she sleeps.  A lot of data.  Yes, I checked all the settings.

Data Booster: OFF

Smart Network Switch: OFF

Keep Wi-Fi on during sleep: ALWAYS ON

Auto-update apps: OFF

Data Saver: ON

And despite this, this is what the data usage on my wife's phone looks like since the start of this billing cycle on the 4/17/2017.

4/17/2017: 10:17pm to 4:17am 0.58004 GB

4/18/2017: 7:47pm to 1:47am 0.43712 GB

4/20/2017: 9:11pm to 3:11am 0.75955 GB

4/22/2017 3:10am to 9:10am 1.39651 GB

It burned through 3.2 GB in a few nights!!!  My phone has no such issues, however.  Our settings are identical, though she has a Galaxy S6 and I have the S7.

I spoke with the Verizon representative and all they can offer is

(1) turn off your Data usage every time you enter your house and turn it back on every time you leave your house because the phone doesn't care what your settings are, it'll do whatever it wants

(2) every carrier does this, so deal with it

Well, if this continues much longer, I'm going to drop Verizon like a bad habit and see if it's true that "every carrier" drains your data whenever it feels like it and if they offer no recourse like Verizon.