UserAgentStringLibrary

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;
        }