"No application running"? Don't forget background processes and services.
In some cases the "traffic" report might be based on ISP<-->network adapter network traffic monitor reports from your network adapter...not whether your adapter has actually transferred meaningful data between the ISP and the computer itself.
I can think of a number of things that might result in what you're seeing...probably a combination.
ISP simply regularly querying connection status to your network adapter (e.g. to determine if it needs to keep allocating resources to your connection, monitoring for network outages/service quality metrics, looking for possible theft of services by monitoring signal strength, etc. Some of that I would suspect could be transparent to the user, but it would depend on exactly what the ISP is doing, whether it bundles multiple tasks into a single query, etc.)
Your internet security application working in the background, constantly monitoring your connection, forcing minimal handshaking/"dialogue" with the ISP to check network status/integrity...making sure nothings been hijacked, tapped into (e.g. by getting reports on signal strength) etc. It would of course depend on just how sophisticated the internet security package is.
Processes checking for updates (although I would think this would appear more as a relatively larger data spike above constant "background" clutter).
Possibly ISP/mail service checking to see if you have any outgoing pending/deferred email to send.
If using an online backup service it might be getting acknowledgments from sending small bundles of data to backup when the computer is not being heavily used...although you'd see both upload and download usage in this case.
If using a USB VoIP adapter (like "magicjack") there might be some continuous low-level ISP<-->VoIP adapter handshaking going on.
And so on.
I don't know if your ISP applies any or all such "background" activity to your quota but I suspect they might.
Of course disconnecting either the ISP-side connection or the computer side connection of your modem would take the values to 0, but even if you software disable the "internet connection" via an internet security application, some
of those activities could continue showing "download" activity as they don't actually result in any effective data transfer to/from the OS itself.
A nice compact network activity monitor/logger that lets you track activity over the last second, last few hours, days, months, with stats for user-selected months, is BitMeter2. It would let you do a rough comparison between your measured network traffic and the monthly quota records of your ISP. For example, assuming I've had BitMeter2 running all the time, I can go back and look at the upload/download traffic stats for each month for the last six months
edit to add:
By the way, if all you want to do is end ALL data transfers between the ISP and your hardware, is do what I do. Insert a quality
coax cable switch or telephone cable switch*
between your wall connection and your hardware. Just switch to a null connector when you want absolute connection security or zero data transfer. That way there is no way the ISP can rack up traffic against your quota from that
or whatever connection you've got going between your hardware and the wall connection...or turn off your wireless adapter. Note that turning off your computer's wireless adapter only provides security...it does not
stop possible handshaking between the ISP and your wireless modem/router/switch. So in theory the ISP might still be counting those transfers toward your quota.
edit to add:
Note also that just disconnecting the cable (ethernet, USB, etc) between your computer's network adapter and your ISP modem (broadband, DSL, etc) or router/switch will not
stop possible handshaking between the ISP and your external modem.
edit to add:
I could also have advised to turn off your broadband modem to provide both complete security and stop all data transfers, but the modem's power-on cycle can take a relatively long time compared to just flipping a switch on the cable on the ISP side of the modem.