Guests generate "Cannot modify header information - headers already sent" error

I'm getting the header error usually ascribed to custom modules or templates with (I've looked at hundreds of forum posts) to extra spaces before opening <?php or un-needed closing ?>

I have triple-checked all files for this issues, yet I continue to get this warning in my error logs-- but it is only for guest access. I am stumped as to why logged in users are not generating the error.

Its not causing any issue on the user end, but it fills my log files and makes it harder to find other errors since there are so many of these.