The favicon is normally the Moodle m for Moodle sites but Nicolas decided to use a raspberry for the MoodleBox.
You will find the favicon when you connect to the MoodleBox via ssh. Using the theme Boost then go to the folder /var/www/moodle/theme/boost/pix and you will see the favicon.ico in this folder. If you want to change the favicon then delete the favicon.ico and upload a new one you want to get. Using the theme Classic then go to the folder /var/www/moodle/theme/classic/pix.
You also can change the favicon using additional html code. Put your new file favicon.ico into a folder on the MoodleBox homepage. Then copy the link to this file and set the additional html code.
<link rel="shortcut icon" type="image/x-icon" href="http://moodlebox.home/pluginfile.php/1073/mod_folder/content/0/f/favicon.ico">