"She kept jumping up on me," she said. "One day she bumped into my chest with her nose. It was unusually sore, and there seemed to be a lump there. I had a fine needle biopsy, but it came back clear."
After more unsettling behaviour from Daisy, Dr Guest had a core needle biopsy and was diagnosed with breast cancer. She underwent treatment and has since been given the all-clear.
"Had I not had Daisy's early warning, I would have had a very poor prognosis," she said. "I have my dog to thank for the fact that I'm standing here today."
That was in 2009, when the dog was just a puppy, and the charity only one year old. Daisy is now the organisation's foremost "advanced cancer dog", able to detect different cancers through smelling a patient's breath, urine or skin.
She is one of a growing number of dogs now being trained as "pattern recognition biosensors" to detect cancer in this way.
The science behind it has been greeted with scepticism by some medical experts, although Dr Guest, 49, as the head of the charity, Medical Detection Dogs, has always been a strong believer in it.
In a demonstration at the charity's base in Milton Keynes, Dr Guest set up a metal "carousel" holding eight urine samples, one of which was from a prostate cancer patient.
Following a command of "seek-seek", Daisy gave each sample a cursory sniff. When she arrived at position five, she stopped, sat, sniffed again, and barked. She had located the cancer.
Interest in the use of dogs to sniff out cancer is growing. It was two doctors, Hywel Williams and Andres Pembroke, who first advanced the idea, in a 1989 article in The Lancet. In 2001 the orthopaedic surgeon Dr John Church — the man responsible for introducing wound-cleaning maggots into the NHS — supported these claims in an article, and then spoke about it on Radio 4.
Dr Guest, a psychologist, heard the programme and got involved. She and Dr Church formed a research team at the Buckingham NHS Trust, with funding from an anonymous philanthropist.
They conducted the first clinically robust, proof of principle study and published the results in the British Medical Journal in 2004.
The sample size was small, but the results were encouraging: the dogs correctly selected urine from bladder cancer patients 56 per cent of the time, compared with the 14 per cent "expected by chance".
Gradually, sceptics are being won over. One is Dr Alan Makepeace, a senior oncologist based at the Mount Vernon Cancer Centre in Middlesex, who has specialised in the treatment of breast cancer for more than 20 years. "Like many clinicians, I was initially very dismissive of the idea of dogs detecting cancer," he said.
After visiting the Medical Detection Dogs centre, however, Dr Makepeace changed his mind. "The data is robust. Dr Guest's passion is driven by good science, not anecdotal evidence. There is so much we don't know about the natural world. A more effective early test could be worth its weight in gold."
Some, however, remain unconvinced. Kat Arney, the science information manager at Cancer Research UK, who has recently visited the Medical Detection Dogs centre, is circumspect.
"It's simply not practical to use dogs on a wide scale for cancer screening across the general population," she said.
"Any test for cancer needs to be as reliable as possible, and the research that's been published about cancer-sniffing dogs so far has had mixed results."
Cancer Research UK points specifically to a small, unsuccessful study carried out in California in 2008. Only two out of six dogs in the first test, and two out of four in the second, performed better at detecting cancerous urine samples than might have been expected by chance. However, the report did conclude that better handling of samples and more effective dog training might produce better results.
In other countries, dogs are already being used by health services. Two British-trained dogs are on active service in northern Italy; when a patient is presenting inconclusive symptoms, the dogs help to determine whether further invasive and uncomfortable tests are needed.
"It would be impossible to have a trainer and specialist dog in every surgery," she said. "But dogs can be used behind the scenes to screen samples. Dogs enjoy it. There's no reason why they can't screen large numbers of samples if they get a biscuit at the end."
The future, however, lies not in animals but in technology. Hossam Haick, an Israeli Arab chemical engineer based at the Laboratory for Nanomaterial-Based Devices in Haifa, has created the 'NA-NOSE', a prototype "electronic dog nose" which could be used to analyse a patient's breath for traces of cancer. Given the pace of technological advance, it is thought that such a device could be introduced within a decade.
The Medical Detection Dogs centre also trains canines for purposes other than cancer detection. People with nut allergies can apply for a dog trained to detect nut traces in their immediate environment. Those with severe type one diabetes can receive a dog that will alert them when their blood sugar is reaching dangerous levels.
A similar service can be provided to sufferers of Addison's disease, a hormonal disease, and the centre also trains dogs to detect bed bugs. The dogs are provided free of charge, but there is a three-year waiting list; in some cases it is also possible to apply to have an existing pet trained.
In total, the Medical Detection Dogs centre has 36 dogs from a variety of breeds, and more are being trained all the time.
* More information about the Medical Detection Dogs and the work they do can be found at www.medicaldetectiondogs.org.uk
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