Hello,
To catch the file, zip the container Folder and when you open the zipped folder, you may see the Threat file in it.
Submit the .zip folder to Symantec Security Response Team on
https://submit.symantec.com/websubmit/essential.cgi
We also offer a self-service site to analyze files, at http://www.threatexpert.com, which can give you more information on the files you submit to it.
Also, check this Article below:
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
Secondly, check these Articles to Answer your Questions on -
What to do when you suspect that a Symantec AntiVirus product is not detecting viruses
http://www.symantec.com/docs/TECH99222
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/docs/TECH98929
Hope that helps!!