Did you recently get an email that said something like:
Howdy! Your site at http://example.com has been updated automatically to WordPress 3.7.1.
The file that is responsible is:
wp-admin/includes/class-wp-upgrader.php
And the function is send_email which is part of the class WP_Automatic_Updater
To disable the email messages you can add this code:
add_filter( 'auto_core_update_send_email', '__return_false' );
For a lot of great options related to managing automatic updates in WordPress 3.7+ see this question.
