TMT may be abnormal in up to 20-30% of patients after a bypass surgery. Positivity may come due to blockage of the grafts or some times even when the grafts are open, small vessel involvement may give a positive TMT.
2. Certainly a person can lead a normal life even with positive TMT, as long as it is not producing symptoms on ordinary exertion.
3. There are a number of tests, like Stress Dobutamine Echo, Stress Thallium Imaging, PET Imaging, CT angio etc., which may give you some idea, whether your grafts are blocked or not. But these are not definitive tests and they do have false positive and false negative results. The only test, which is full proof is coronary angiography. You must remember that almost 5-10% of grafts get blocked within first one year after bypass surgery and therefore if a person has either recurrence of symptoms or as you have written, a strongly positive TMT, then one must undergo coronary angiography to have a look at the grafts and where exactly are the blockages located. However, if the TMT is positive in the fourth stage only and you have no symptoms and carrying on all activities, then you may decide to pursue aggressive medical management and risk factor control and not probe things any further. However, if you want a full proof answer, then certainly a coronary angiography is mandated.
I think this is about the best we can give you by way of answers, but if you want any more information, I suggest you should consult your treating cardiologist, who would be a better person to answer them.