Hmm, I think I sorted out from where this notice is coming from. It looks like responsible for this is the latest version of Akeeba Admin Tools Pro (maybe it's also in the free core version don't know as I don't use that).
Admin Tools Pro v4.1.0 have some new features in its WAF configuration:
1. Monitor Global Configuration
When this is enabled and someone tries to change the Global
Configuration of Joomla!, either from the back-end or the front-end,
you will either be notified or they will get blocked (depending on
your settings below).
2. Monitor Component Configuration
When this is enabled and someone tries to change the configuration of
any core Joomla! or third party component (what you see when you click
Options in a component's toolbar) from the back-end of your site you
will either be notified or they will get blocked (depending on your
3. Action for configuration monitoring
What do you want to do when either global or component configuration
is enabled and a change is detected in the configuration. 'Email' will
simply send a warning email to the email addresses you've configured
to receive security exception emails and only if you have configured
such email addresses. The changes in configuration will go through.
'Block' will treat these changes as security exceptions. The changes
in configuration will NOT go through.
This is actually the action that Admin Tools should do when something from the above 2 things happens. One of the options is to send an email.
So if any of the 1 and 2 features above is enabled and the email option is selected as action, but there is no email recipient configured to receive Admin Tools Security Exceptions, then each time someone saves anything in the Global or Component Configuration, Admin Tools will try to send an email but it won't have a recipient.