wcmaster on "Apache Error Logs filled with signuppageheaders Warnings"

A website's Apache error logs is filled with the following PHP Warnings:

call_user_func_array() expects parameter 1 to be a valid callback, function 'signuppageheaders' not found or invalid function name in /wp-includes/plugin.php

These warnings are coming from various IP addresses. It seems to me people are trying to find exploits within WP.

• Should I be worried these people may find a hole to exploit?
• What exactly are these people trying to do?
• Is this possible exploit already been locked down from previous updates?

Thanks for your help. None of my searches on various forums showed anything like this.