I looked at line 739 in Loader.php. It's
$buffer = ob_get_contents();. Here is the code right around it.
log_message('debug', 'File loaded: '.$_ci_path);
// Return the file data if requested
if ($_ci_return === TRUE)
$buffer = ob_get_contents();
Looks like a debug message is being logged right before this line which will help you track this backwards.
Here's details on enabling logging in EE config.php.
You could also add an additional debug message inside the conditional. Something like:
log_message('debug', 'Buffer data: '.$buffer);