what does it mean: a threat in "System Volume Information"?

I have met with this annoyance many times and I decide to take some time to
investigate this problem:

When I luanch my Symantec Antivirus 10.1 coporate client to do a full scan
of all my computer, it did not raise any alarm or detected anything.

But after a few hours, the "Auto Protect"(which was enabled and supposed to
protect me real-time) , due to unknown reason, raised an alert that there is
a file in "System Volumn Information" folder that was infected and got
deleted. But that file had always been there.

It was always like this: when you do a full scan, it did not detect
anything; but the real-time protect seemed to have more power and it can
detect something from time to time.

What's the problem?


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