In production mode, Rails will not be responsible for serving static assets. Therefore, you are getting this error. Thin won’t do it either, since it’s just a wrapper around Rails.
This is controlled by this setting in config/environments/production.rb in your application:
config.serve_static_files = false
Or in Rails 5:
# config/environments/production.rb
config.public_file_server.enabled = true
Or set ENV['RAILS_SERVE_STATIC_FILES'] to true.
You can either set to that true or use a real server like Apache or Nginx which will serve the static assets. I suspect Pow may also do it.
If you’re on Heroku, they recommend the use of the rails_12factor gem which enables this setting by default. Place the gem into a production group in your Gemfile, like this:
group :production do
gem 'rails_12factor'
end