SEP (Symantec Endpoint Protection) "deferred scanning" feature may result in delay between file write and risk detection, up to many hours in some situations.
SEP's antivirus/antispyware includes a "deferred scanning" feature designed to preserve system performance in the case of large disk I/O operations during file WRITE operations for large files or many different files, or in the case of re-scanning a file after definition update.
It is important to keep in mind that these deferred scans do not mean a decrease in security; any READ/EXECUTE operations will still trigger an immediate scan and prevent any potential risks from propagating during the wait for a deferred scan.
A deferred scan may result in a significant delay (up to many hours) in detections, for example, in cases where a large file or archive of many different files is copied to disk without any further operations. Again, there is no need for concern as there will be immediate scans and detections if there is any attempt to manipulate the files further.
There may be situations where deferred scans are suspected of interfering with other file operations and they may be disabled for troubleshooting purposes if necessary. This is not recommended as a permanent change since it may lead to unacceptable performance during the types of operations described above. See How to disable deferred scanning in Auto-Protect for Symantec Endpoint Protection.