And then there's The Strange Case of My Piper, who has been known to have drinking/peeing issues. Nothing wrong with blood work. Nothing on the x-rays. Nothing in the urinalysis. Antibiotics help. Although...we just went through two rounds of this when our usual antibiotic didn't work, and the second did, but made her sick. She too is currently eating beef because that's the only thing she would eat while on the antibiotics, and I want to get some weight on her before I try her food again since she has also been known to get a UTI after a food change.
ETA: She also leaks a lot more when she's having her drinking/peeing bouts. She's slightly incontinent (spotting) anyway; she's far worse when she drinks too much.
I hope the urinalysis provides some answers for you and your foster.