Unfortunately Everest help isn't that illuminating
"18. What does the "Aux" temperature shown on the Computer / Sensor page mean?
There is no standard for sensor registers layout, so the "Aux" temperature could show the temperature of the CPU or the motherboard, or might be a non-connected wire of the sensor chip, and so it could show a bogus value. "
I looked on my Gigabyte board PC and it shows up on that one to, so it would appear to be a feature of the board/brand rather than a non connected wire. I then checked out the mobo manual and it doesn't give any hint either, so you may need to contact Gigabyte themselves to see what sensor is involved.