@jpbmass 

All devices do this in some way, its not specific to your device.

Basically, the router will burn through a few MBs every few hours just as background usage, even if nothing is connected.   Its usually the following reasons

1. Device is checking for an internet connection - the router will hit random sites periodically to confirm the internet is working.   This may only be a few kb each time but it mounts up

2.  The device is checking for firmware updates

3. Carrier status reports - some carriers will poll the devices for data (model, connection, speed etc) on a semi regular basis, this also eats some data.

Personally, wouldn't worry it's totally normal.   My Netgear 4g dongle eats around 10-12 MB per day if I have powered on and idle.   Only know this as I asked the same question you are years ago  :)

@Philbert Thanks -- I guess that makes sense.  I'm changing from ATT to TMobile to see if I get better reception -- will be interesting to see if this background usage changes.