The debug messages are logged into the var/log/debug.log file

[dmitry_fedyuk (Dmitrii Fediuk)]

https://github.com/magento/magento2/blob/2.0.0/lib/internal/Magento/Framework/Logger/Handler/Debug.php#L16-L16

[dmitry_fedyuk (Dmitrii Fediuk)]

See also:

• How does \Magento\Framework\Logger\Monolog::critical() work?
• How does \Magento\Framework\Logger\Monolog::addRecord() work?
• The exceptions are loggged into the var/log/exception.log file
• The non-exceptional messages are loggged into the var/log/system.log file
• The default implementation of the \Psr\Log\LoggerInterface is \Magento\Framework\Logger\Monolog
• How to write a message to a log file?