Solved. Unexplained data usage

I’m running a RUT240 which uploads data to my MQTT broker. Data usage is off-course determined by the upload frequency and message size.
I was uploading every 10 minutes, and came to a data usage (based on the cost my provider calculates) of 100kb per cycle, which seems unrealisticly high.
Just now, my balance of my provider dropped by 2,44 euro’s, which would mean 24,4 MB was transmitted/received in a matter of minutes. That can’t be right, how can i check in the RUT240 what data was processed? I haven’t enabled any logging so far.

Luckily the connection is established through my VPN server which logs the packet headers. Once i saw ‘Android TV’ it hit me: the smart tv near the RUT240 has its network credentials stored, so probably the smart tv did an update check or something.