What does a single beep and one flash mean when using my Ames non contact tester
Non contact tester will flash over nothing - tap your palm - flashes, slides across the dashboard - flashes, falls out of your shirt pocket, flashes as it hits the floor, and never works again.  

It's important to remember that non contact voltage testers are simply not dead solid reliable testers.  

There are lots of circumstances that will give you a false negative - it will fail to detect voltage present.  There are not many circumstances where it will give you a false positive - usually it it lights up and stays lit it is seeing something.