Looks like it uses the same api as Rapportive based on the permissions it asks for when installing.
No, it is using FullContact APIs.
Oops.. you are right. They are using rapportive.
var LOGIN_URL = 'https://rapportive.com/login_status';
var LOOKUP_URL = 'https://profiles.rapportive.com/contacts/email/';
var API_HOST = '104.131.218.68';
var API_URL = 'http://'+API_HOST+'/api/profiles';
How do you access the source?
After installing the extension, open the extensions menu, find it and click "Inspect views: background page", then check the JS file that it references.
Any chance you can share the source in a gist or something?