Turns out this happens in very small amounts as phones with LTE ON oscillate between 3G for calls and back to LTE. At least, according to this post: https://android.stackexchange.com/questions/82837/4g-mobile-data-being-used-even-when-mobile-data-se...
I find this hard to believe. Is this like the WiFi assist on iOS?
I don't know. But I have seen data used to make the initial data connection when turning cellular data ON.
Sometimes the wifi connection happens after the cellular connection. A bit of data can sneak in before wifi disables the cellular data. This article seems to point to a condition where this is continual long after the intial transient during phone start up.