What could cause an indoor cat without clinical signs to be chronically neutropenic?

Recently, we had a case at our hospital...chronically neutropenic... Felv/fiv test were negative. The cat was also chronically anemic as well. As a last resort, we did a bone marrow aspirate. It was positive for FELV by IFA. We recheck an elisa snap felv/fiv test to see if the first test had been a mistake, and it was still negative. We treated the cat with vitamin suppliments, epogen for the anemia...and to this day he is doing very well...