Unfortunately, I've got a couple of viruses with what was supposed to be great protection. One of the times was when I had Norton. I called them finally & they scanned the pc remotely & found the virus. Then they said it was too intelligent to be detected, that's why it was there. They continued to charge us $100 to remove it.
You might want to have it looked at by a pro, that may be the only way to find it.