Well, we have a turnaround on Lea. I finally got access to her medical records (the previous owners had to add me to the account; these vets extend HIPAA-like privacy to pets!) and it turns out that she *had* been in for a visit in 2006 and she *had* been given the FIV vaccine at that time. So the test was a false positive -- or, I should say, it correctly detected FIV antibodies but they were present because of a previous vaccine, not an infection. This is certainly a relief. I'll wait until they send me the paperwork to confirm, but it looks like we can allow her outside after all (though I'm happy to keep her as a mostly-indoor cat anyway).