Firstly, I’d like to extend my deepest sympathies – losing a pet is never easy and is especially difficult in such circumstances. There are a few possible reasons for inexplicable sudden death in cats, including obvious causes such as cardiomyopathy (heart disease) and heartworm infection. Both of these can result in instant death with no previously apparent symptoms. Sometimes other serious illnesses have symptoms so subtle that they are missed by both veterinarians and their owners, such as feline leukemia virus, liver failure and kidney failure. However, it is highly unlikely that any of these would result in the blood that you describe. A brain hemorrhage (for which there is likely to be other underlying causes) could result in sudden death and could result in bleeding from the facial orifices, but this is highly unusual.

This leaves two other possibilities: poisoning or trauma. Two toxins that can cause sudden death and are relatively common are Zinc Phosphide (a common component of rat poison) and Permethrin (found in flea control products for dogs only). Permethrin poisoning does not usually result in bleeding from the mouth but it is possible that the blood was contained in vomit from the stomach. Zinc Phosphide however does not normally induce bleeding or vomiting- the most obvious signs are twitching, walking abnormally, muscle tremors and seizures. I suppose it is also possible that your cat came into contact with a poisonous toad and the bleeding could be an indication that your cat was over-scratching his irritated mouth in response to the stinging feeling.

Bleeding from the mouth and/or nose preceding sudden death normally indicates internal injury, possibly caused by extreme trauma. Is it possible that your cat could have been hit by a car before moving to the back of your house and then being unable to continue? Without being able to examine your cat, this seems the most likely explanation to me.