I have two leakers: Minnie and Cora.
Minnie has spay incontinence. It showed up when she was just 3 and was being treated with maximum doses of prednisone to try to control her inflammatory bowel disease. She often woke up in a huge puddle, usually (not always) in mid-morning. Since she was in so much trouble already, I didn't want to add another drug, so I protected the furniture, and, during the worst period, had her wear home-made diapers. Now that she's off pred, she sometimes spots. The furniture is still protected, and the covers are washed every couple of weeks. When I notice a spot, I spray it with enzyme cleaner.
Cora has a problem similar to Zema's: late afternoon binge drinking, and occasional substantial leaks. The drinking/leaking link is not as strong, and I have not yet determined whether it is related to food. In the meantime, we're treating her with Propalin, which has controlled her leaks.
Since Kate has just had a UTI, I'd be inclined to suspect that bacteria or irritation are still present, even if nothing shows up on tests.
If you conclude she is incontinent, there are now several alternatives to PPA. Several are mentioned in a recent thread here.