To be clear, she is one of my co-workers, not a medical researcher. But she has been having bad test results, too.

It seems to me that a likely explanation is that people who have a heart attack, but no underlying diagnosable risk conditions, are less likely to make lifestyle changes or take new medication. Thus, they don't change whatever it is that led to the first heart attack. Whereas people with a diagnosable condition are encouraged to make more changes. But this doesn't seem to be a known thing.