Symantec Endpoint Protection (SEP) for Mac may detect many files as infected but result is "Left alone" even though "Auto quarantine" is enabled.
This behavior is as designed. Many computer file formats are archival types (e.g. compressed files, client email stores, database files, etc) and threats that are detected may actually be items within the archive. SEP cannot safely delete or quarantine individual items in some archival formats, and chooses to leave such files alone rather than taking the riskier action of deleting or quarantining the entire archive. A manual scan of an archive may not detect any threat because the format is proprietary and can only be scanned when mounted or opened by the supporting operating system or application.
Threats that are specific to other operating systems may also be left alone, since the threat detected does not affect Mac OS and it may be riskier to delete a file that another operating system may depend upon.
It is recommended that "Left alone" instances be examined on a case-by-case basis and deleted manually only after careful consideration.