Continual background data usage

Hello,

When my laptop is idle, with no (visble) apps running, there is a continuous ~2kB/s incoming data over the wifi, according to the system monitor. (See screen grab below).

I've run nethogs, which is not showing any data at all.

NetHogs version 0.8.1
    PID USER     PROGRAM                                            DEV        SENT      RECEIVED       
      ? root     unknown TCP                                                   0.000       0.000 KB/sec

  TOTAL                                                                        0.000       0.000 KB/sec 

Is there another way to find out what is using the data? All apps are closed, and I've turned off dropbox too.

Thanks.

Some other useful commands:

lsof | grep TCP.*ESTABLISHED

sudo iftop -i [interface]

Usually when I see that kind of activity attributed to unknown TCP, it's because it's data being routed from my machine.
As in: I have a machine with a connection of type "shared" such as a Wi-Fi hotspot or a wired subnet on 10.42.0.1, and one machine on the local subnet is sending data outside, so the data is being routed by the machine showing "unknown TCP"

Edit: obviously not the case for you since your upload is not mirroring the download, but it's also possible the data is routed through localhost (127.0.0.0/8)

Edit2: if you're on a public Wi-Fi it can also be multicast traffic, I see a lot of that on my residence's network.