Why does Symfony still log to a dev.log file, even when I didn't define it in a loghandler?

REMOVE monolog.handlers.mainfrom config_dev.yml.

It usally contains path: "%kernel.logs_dir%/%kernel.environment%.log"

config _dev.yml (default)

monolog:    handlers:        main:   # <- remove this handler            type:   stream            path:   "%kernel.logs_dir%/%kernel.environment%.log" #<- logs/dev.log            level:  debug

Remove the main handler from this config file.

If anyone comes across this and is still interested in why this happens, the debug handler is injected in the \Symfony\Bundle\MonologBundle\DependencyInjection\Compiler\DebugHandlerPass::process() method...

class DebugHandlerPass implements CompilerPassInterface{    // ...    public function process(ContainerBuilder $container)    {        if (!$container->hasDefinition('profiler')) {            return;        }        if (!$container->getParameter('kernel.debug')) {            return;        }        $debugHandler = new Definition('%monolog.handler.debug.class%', array(Logger::DEBUG, true));        $container->setDefinition('monolog.handler.debug', $debugHandler);        foreach ($this->channelPass->getChannels() as $channel) {            $container                ->getDefinition($channel === 'app' ? 'monolog.logger' : 'monolog.logger.'.$channel)                ->addMethodCall('pushHandler', array(new Reference('monolog.handler.debug')));        }    }}

As you can see, this pushes a new handler on to every registered channel, thus overriding any other handlers that might already have been added.