Nginx – How does PHP5-FPM send errors to nginx

I've been experimenting a bit with error logging in php-fpm and nginx since I couldn't find any good explanation on the web. Most guides said I should change catch_workers_output to yes if I want to send errors from php5-fpm back to nginx. However, during my experiments I've found that even when catch_workers_output is set to no, errors are still being logged properly by nginx.

Here's my virtualhost config:

server {                                                                                                                                                                                                                                 
        server_name     domain.com;
        return  301 http://www.domain.com$request_uri;
        access_log off;
}

server {
        listen 80;
        listen [::]:80;

        root /home/websites/domain.com;
        index index.php index.html index.htm;


        error_log /home/websites/logs/domain.com/error.log warn;
        access_log /home/websites/logs/domain.com/access.log;

        #switch on gzip
        gzip on;
        gzip_min_length  1100;
        gzip_buffers  10 32k;
        gzip_types    text/plain application/x-javascript text/xml text/css;
        gzip_vary on;


        location / {
                try_files $uri $uri/ /index.php?q=$uri&$args;
        }

        location ~* .(gif|jpg|jpeg|png|css|js|ico)$ {
                expires 30d;
                access_log off;
        }

        error_page 404 /404.html;

        location ~ \.php$ {
                fastcgi_split_path_info ^(.+\.php)(/.+)$;
                # NOTE: You should have "cgi.fix_pathinfo = 0;" in php.ini

                # With php5-cgi alone:
                # With php5-fpm:
                fastcgi_pass unix:/var/run/php5-fpm.sock;
                fastcgi_index index.php;
                include fastcgi_params;
                fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        }

        # deny access to .htaccess files, if Apache's document root
        # concurs with nginx's one
        location ~ /\.ht {
                deny all;
        }
}

Here are my findings:

Exp 1
        poolconf:
                ; catch_workers_output = no (commented out)
                php_admin_value[error_log] = /var/log/fpm-php.www.log
                php_admin_flag[log_errors] = on
                ; php_flag[display_errors] = 0

        result:
                errors not shown in browser
                error written in /var/log/fpm-php.www.log
                error not written in virtualhost error-log in nginx
                errors not displayed in stderr when running php5-fpm non-daemonized

Exp 2
        poolconf:
                catch_workers_output = yes
                php_admin_value[error_log] = /var/log/fpm-php.www.log
                php_admin_flag[log_errors] = on
                php_flag[display_errors] = 0

        results:
                no error in browser
                error written in /var/log/fpm-php.www.log
                error not written to virtualhost error-log by nginx
                errors not displayed in stderr when running php5-fpm non-daemonized

Exp 3
        poolconf:
                catch_workers_output = yes
                ; php_admin_value[error_log] = /var/log/fpm-php.www.log
                ; php_admin_flag[log_errors] = on
                php_flag[display_errors] = 0

        results:
                no errors in browser
                error  NOT written in /var/log/fpm-php.www.log
                error WRITTEN to virtualhost error-log by nginx
                errors DISPLAYED in stderr when running php5-fpm non-daemonized

Exp 4
        poolconf:
                ; catch_workers_output = no (commented out)
                ; php_admin_value[error_log] = /var/log/fpm-php.www.log
                ; php_admin_flag[log_errors] = on
                php_flag[display_errors] = 0

        results:
                no errors in browser
                error NOT written in /var/log/fpm-php.www.log
                error WRITTEN to virtualhost error-log by nginx
                errors NOT displayed in stderr when running php5-fpm non-daemonized

My question is how does PHP5-FPM send error logs to nginx even if there's no stderr output (when catch_workers_output=no) from php-fpm? I cannot find it documented anywhere.