Why is my MacBook sending out data, even when idle?
It could just be bonjour or some other background network process or daemon communicating. You don't have to be explicitly browsing the web to be using network traffic 

Bonjour - Is a set of networking protocols to allow different machines to see each other with zero configuration.

Daemon - Is just a process that runs unattended in the background.

Several hundred bytes is a tiny amount of traffic, it's nothing significant.  If you would like to monitor which ports are open, use "netstat" at the terminal