You are correct, WordPress checks for updates to core and plugins every 12 hours, but a better way to word it would be: it checks updates if last update was more than 12 hours ago.
The 12 hour setting is hard codded in wp-includes/update.php
The last updated dates are stored in wp_options table and the options are:
_site_transient_update_core
_site_transient_update_plugins
_site_transient_update_themes
Because this check does not happen precisely after 12 hours, but rather next time the condition is met (at least 12 hours has passed) then you will not miss the update.