heartcombo--devise/lib/devise/hooks/forgetable.rb

# Before logout hook to forget the user in the given scope, if it responds
# to forget_me! Also clear remember token to ensure the user won't be
# remembered again. Notice that we forget the user unless the record is frozen.
# This avoids forgetting deleted users.
Warden::Manager.before_logout do |record, warden, options|
  if record.respond_to?(:forget_me!)
    record.forget_me! unless record.frozen?
    cookie_options = Rails.configuration.session_options.slice(:path, :domain, :secure)
    cookie_options.merge!(record.cookie_options)
    warden.cookies.delete("remember_#{options[:scope]}_token", cookie_options)
  end
end