Nowadays rootkits are on the rise so is symantec on detecting them.SEP does detect most of the rootkits that are there in the wild.Detection capabilities of any antivirus depends on its Honey Pots (Sensors) across the globe and Symantec has the highest number of Sensors across the globe.
But since there are so many in the wild that it is not possible to detect all them untill and unless symantec has a sample.So if you are facing any kind of Rootkits issue ..Open a case with Symantec or submit your Virus Sample to https://submit.symantec.com/gold 
To double check if there is really a rootkit on your computer you can scan using Microsoft's Rootkit Revealer.