Sinus tachycardia is not a diagnosis. It's a normal (but rapid) heart rhythm.
Sinus tachycardia is caused by increased sympathetic nerve activity,and usually the reason is obvious, like anxiety, exercise, stress, infections, etc. However, sometimes the reason is more serious, like heart failure, intoxications, severe dehydration or heat stroke (to mention a few). Thyroid diseases (hyperthyroid) can also cause sinus tachycardia.
In other words, sinus tachycardia is no disease or diagnosis, but it can be a sign of something more serious. But usually it's not.
I think my point was: Every person alive have at some point experienced sinus tachycardia. It's no need to mention it to the insurance company, as it's not a disease.