either you had the virus but cleared it on your own as 15-20% of people do...
or your original antibody test was a false positive, which does happen.
I'd continue to get rechecked with the tests you've been having, say once a year for a couple years...but if you remain undetected all that while then I'd go out five years.
It's totally possible for there to be machine/human error or slide contamination.
If they could not detect any RNA when your antibody test was positive, the chances are you don't and probably never did have the virus. If there are antibodies there should be detectable RNA.
This answer was given by the doctor who actually invented the NGI_PCR machines that detect the virus on this forum, so I'm assuming he knows of what he speaks.
mb