The Magento 2 module will log any not found page request on your website. You can view logs at the System -> Tools -> 404 Error Log.
The log cleans by a cron task which can be configured at the admin panel Stores -> Configuration -> The S Group -> 404 Error Log
Run the following command at Magento 2 root folder:
composer require thesgroup/module-not-found-url-error-log
php bin/magento setup:upgrade
php bin/magento setup:di:compile
php bin/magento setup:static-content:deploy
Run the following command at Magento 2 root folder:
composer remove thesgroup/module-not-found-url-error-log
php bin/magento setup:upgrade
php bin/magento setup:di:compile
php bin/magento setup:static-content:deploy