Monkey-patch Rails I18n.translate to see what keys ActiveModel is trying to use.

i18n.rb

#initializers/i18n.rb
I18n.module_eval do
  class << self
    def translate_with_puts(*args)
      Rails.logger.debug "#{args}"
      old_translate(*args)
    end
    alias :old_translate :translate
    alias :translate :translate_with_puts
  end
end