The method Zend_Loader::isReadable() uses fopen to check if a file exists and the @-operator to abort its error-messages:

public static function isReadable($filename)
{
    if (!$fh = @fopen($filename, 'r', true)) {
        return false;
    }

    return true;
}

This only aborts the messages from beeing displayed, the error-handler will be triggert anyway. So if you use your own error-handler, everytime a file is beeing checked with this function and not found (and this may happen very often) a php-notice is thrown and beeing logged into your logfile for example or your database.

Why not use the 'file_exists()' function of php?

// example with include path:

static $paths = null;
if ($paths == null) {
    $paths = explode(PATH_SEPARATOR, get_include_path());
}
    
foreach ($paths as $path) {
    $fullpath = $path.DIRECTORY_SEPARATOR.$filename;
    if (file_exists($fullpath)) {
        return true;
    }
}
      
return file_exists($filename);