Yes, you could have a false positive for a number of reasons. Both sides of my neck lit up like a Christmas tree, but not one single node tested positive for cancer (they took 45) from either side.

The most common cause is food, but your individual body chemistry can be the cause as well. In my case, as I had been losing weight, my surgeon did not want me to go 24 hours without eating, and my tumor (on my oral tongue) was so painful to touch that I was down to a mostly liquid diet - making it impossible to avoid carbs leading up to the test.