Here is one way to find out what applications are connected over network.
If you enable developer mode and gain access to terminal, write devel-su in terminal to become root (which will ask the password set when enabling developer mode via settings) and then write
netstat -ntupce
in terminal to see the status of your network connections. If you want to display the hostnames instead of IP addresses use
netstat -tupce
netstat will list the programs with their process ids and both local and foreign addresses they are connected to. With the c parameter the list is continuously updated. This will help at least to find out the program that sends data, as send queues and receive queues are also shown. Also, the user identifier is shown, if it is 0 then it is root (system), 100000 is you, the Sailfish OS user (corresponds to user nemo on the device). To exit netstat press Ctrl+C.
If the application is not shown in the listing and traffic still goes out, then it needs to be inspected further.
The listing usually contains connmand (connectivity daemon) and systemd (system daemon), which are Sailfish OS system services.
NOTE: Seeing the printout of netstat is easier at the larger terminal of a computer (rather than on the tiny screen of the phone). Consider SSH connection over USB: Windows and Linux/Ubuntu.