Cannot believe there was a silent mi. How unlikely is it that diagnostic tests for heart attacks are correct?

Extremely likely. 30% of all heart attacks are silent or unrecognized. The number is 50% in people with diabetes. Modern blood sampling for the diagnosis of and acute heart attack (myocardial infarction) is >99% accurate and the number is further increased if there are ekg and/or echo changes. (accuracy is diminished if the event occurred remotely e.g. >10 days ago).