It is at WebWorkContext.CurrentCustomer's part.

                //check whether request is made by a search engine

                //in this case return built-in customer record for search engines

                //or comment the following two lines of code in order to disable this functionality

                if (customer == null || customer.Deleted || !customer.Active)

                {

                    if (_userAgentHelper.IsSearchEngine())

                        customer = _customerService.GetCustomerBySystemName(SystemCustomerNames.SearchEngine);

                }

and to _userAgentHelper.IsSearchEngine()

        /// <summary>

        /// Get a value indicating whether the request is made by search engine (web crawler)

        /// </summary>

        /// <returns>Result</returns>

        public virtual bool IsSearchEngine()

        {

            if (_httpContext == null)

                return false;

            //we put required logic in try-catch block

            //more info: http://www.nopcommerce.com/boards/t/17711/unhandled-exception-request-is-not-available-in-this-context.aspx

(I checked this page, but the url is not valid now)

            bool result = false;

            try

            {

                var uasParser = GetUasParser();

                var userAgent = _httpContext.Request.UserAgent;

                result = uasParser.IsBot(userAgent);

                //result = context.Request.Browser.Crawler;

            }

            catch (Exception exc)

            {

                Debug.WriteLine(exc);

            }

            return result;

        }

and to var uasParser = GetUasParser();

        protected virtual UasParser GetUasParser()

        {

            if (Singleton<UasParser>.Instance == null)

            {

                //no database created

                if (String.IsNullOrEmpty(_config.UserAgentStringsPath))

                    //UNDONE: maybe, throw an exception?

                    return null;

                var filePath = _webHelper.MapPath(_config.UserAgentStringsPath);

                var uasParser = new UasParser(filePath);

                Singleton<UasParser>.Instance = uasParser;

            }

            return Singleton<UasParser>.Instance;

        }