Hypertrophic Cardiomyopathy (HCM) is found in all breeds, not just Sphynx. HCM is the most common heart disease found in all domestic cats. This is a genetic disease that can happen at any time to any breeder regardless of scanning and precautions. HCM is a thickening of the left ventricle wall of the heart. Many cats that have heart conditions show no symptoms (ie heart murmur) when checked by a non-specialist vet.
Some breeders are proactive and scan their breeders annually at board certified cardiologists to ensure they are breeding healthy cats. This is the best tool we have right now, however it is not a guarantee because the gene has variable expression and can express itself at any time. Make sure the breeder you are working with scans yearly!