Hi, Jeff. There are a few studies in regards to this phenomenon. You can go to PubMed and do a search on the topic.
[Thymectomy and antibodies to acetylcholine receptors (AChR) in patients with myasthenia gravis in the early postoperative period]. - PubMed - NCBI
This one only refers to the "early postoperative period" and not what may happen a year or more later, as what happened with Lisa.
There is the possibility that the antibodies are always in the body. But when the thymus is removed, the antibodies are now "circulating antibodies" and not "tissue bound" ones. In other words, they are there but not attacking the NMJ anymore.
I haven't run into any research that has been able to elucidate this process to any degree of certainty. More studies are being done recently, so stay tuned!
Lisa is right in that the number of antibodies do not necessarily relate to the severity of MG. Trust how you are feeling. If you become weak, just call your neuro!
I'm glad that you (both) had such success!

Annie