Hello,
First of all , I would recommend that you should have the latest definitions on your machines.
Again, Yes. the Suspicious Threat will be detected. Read the Below Article:
Using Symantec Support Tool, how do we Collect the Suspicious Files and Submit the same to Symantec Security Response Team.
https://www-secure.symantec.com/connect/articles/using-symantec-support-tool-how-do-we-collect-suspicious-files-and-submit-same-symantec-sec
Symantec Support Tool is not a Scanning Tool, however, it may find the suspicious Files from your machine and may assist you Submitting the unknown and undetected files by Symantec as well.
Once you know the suspicious files, you could upload them to the Symantec Security Response Team and they would assist you with the same.
Well, in regards to why Symantec not detecting that Threat, you may read the Symantec Knowledgebase Article below:
Scanning a file with a competitor's antivirus program detects a virus, but scanning with Symantec AntiVirus or Symantec Endpoint Protection does not
http://www.symantec.com/business/support/index?page=content&id=TECH98929&actp=search&viewlocale=en_US&searchid=1301410542550