However, the best approach is to take X-rays and do an ultrasound exam, as they each have some strong points and having the information from both is useful. Despite thinking this way, most of the time when I see X-rays with excessive gas accumulation I want to do exploratory surgery, even in older sick cats. My reasoning is that most intestinal obstruction problems get worse rapidly and delaying an attempt at treatment just makes it more likely that removal of a portion of the intestine will be necessary. If there is not an obstruction most cats recover from surgery very well, even if they are older kitties. I would not be put off by mild rises in BUN or creatinine, nor would minor anemia bother me in this decision making.
Cancers can and often do show a very rapid onset of clinical signs.
I don't think that the cancer has suddenly appeared in these cases, I think it has suddenly managed to do something the body can't tolerate, like block a bile duct or block an intestine. In a cat in your cat's age range cancer has to be among the list of possible problems and usually makes the top of the differential list. Some cancers are removable and surgical biopsy can give information on the type of tumor that allows for making an accurate prognosis and better treatment plan. This is not always a part of routine blood screens. If it wasn't checked it would be a good idea to ask about doing so.