Mobile data is turned off but provider shows data trickle I have turned off cellular data on my phone (Nexus 5, Android 6.0.1) but the data usage summary on my provider's portal is still (slowly) increasing.

Does Android still use a small amount of data even when told not to, or is it provider shenanigans?

Provider is Public Mobile.
If you're using LTE, the reason could be, that LTE is causing traffic even if mobile data is turned off.
See here: https://android.stackexchange.com/a/132642/185648