No, they bloody didn't. They just didn't. 'Cats detected my cancer', reads a headline on Mail Online. But, let's face it, they didn't at all.
The ways in which this story is nonsense are obvious. Stephanie Doody, the woman in question, says (and, I'm sure, believes - and may even be right) that her three cats, whose names are not important but are dutifully reported in the story because of Human Interest, started behaving oddly and prodding her stomach. This, she also says and also no doubt believes (but in this case she is wrong) alerted her to the rare tumour on her appendix, a type which usually goes undetected until it is too late.
Which is all very heartwarming and so on, but look. The cats apparently started acting oddly in February. She didn't seek medical attention until July. And the story itself says that the reason she went to the GP was not because her cats were prodding her stomach ('Excuse me, doctor, but Tiddles just slapped me on the abdomen and I was wondering if that means I have cancer') but because she'd 'found a small lump after losing some weight'. So, less to do with mystic cancer-smelling cats and more to do with, you know, having symptoms.
The 'my cat/dog/guinea pig/alpaca smelled my cancer' trope is well-established, but it is, as far as I can see, codswallop. There is some very slight evidence that trained dogs, in controlled conditions, can detect a difference between the urine of patients with cancer from those without. But it's far from clear that the dogs are smelling cancer-specific chemicals, rather than odours given off by ill health in general.
And those are trained dogs. The idea that a bog-standard house cat will hoist up its nose and say 'ooh, that smells a bit cancery to me' is ludicrous. The far more plausible explanation is that, if the cats really did change behaviour at all, it was for a more prosaic reason (Mrs Doody changed her perfume, or the cats just changed their behaviour because they're cats), and then, after the fact, Mrs Doody assigned that change in her memory to the cancer, because we always look for meaning and pattern in everything even when it's not there.
Maybe this doesn't matter, but I think it does. For a start, it raises the possibility of GP surgeries being flooded by people who are perfectly healthy but whose cat has recently started headbutting them. Second, it gives false reassurance to other people who aren't healthy, who might ignore real symptoms if their dog isn't jumping up and down. And in general, pushing cute but hippy-dippy ideas about health and cancer self-diagnosis into the public consciousness just isn't a responsible thing to do. It seems a short walk from saying cats can detect cancer to claiming that the laying on of hands can cure it.
In short: if your cats start behaving oddly, don't go to the doctors, you'll only get laughed at. If, on the other hand, you lose some weight and find a lump, do, whatever your cats are doing. And ignore stories about cats smelling cancer, for god's sake.