Zend certified PHP/Magento developer

Magento 2 static asset response code when static deployment is running

When you are generating static content and you view a link to an asset for example

https://domain/static/version1234/frontend/Vendor/Theme/en_GB/mage/translate.js

If you view the file before it’s been generated you see the following error

Service Temporarily Available

And a 404 response code is sent. Refresh after static content process is finished you see the asset and a 200 response code is sent. Does anyone know where the code that sends the 404 response code is set?