A lot of it boils down to blood pressure. High blood pressure is a serious contributing factor to cardiovascular incidents (as well as a slew of other negative health risks), and getting a good night's sleep will help keep blood pressure down. This is also why the amount of heart attacks are up around 24% after daylight savings[1]; an hour less sleep means higher blood pressure means higher risk of heart attack (relative to any other 'normal' day).
I can definitely see how the same logic could apply to Mondays. Less sleep, more stress = higher blood pressure = higher risk of heart attacks.
[0] https://www.goodreads.com/book/show/34466963-why-we-sleep
I disagree. It's not that the symptoms aren't there, but that they have become normalized due to obesity, smoking, etc. being commonplace. Shortness of breath, sleep apnea, feeling weak, upper body tension/pain, etc. are usually present for quite a while in most people before it finally happens. People don't check their blood pressure often enough despite it being so cheap and easy to do.
Cholesterol, Fasting Insulin levels, and (if male) free Testosterone are other good ones. Cholesterol and Insulin should be checked by all adults annually, and BP should be checked at least annually. T isn't checked as routinely, but it's worth knowing where you fit and has an impact on your metabolics and the test isn't a big deal.
Also, resting heart rate is very easy to measure, especially if you have any kind of fitness/smart watch, and that's a good marker of health too.
I'm a fan of getting lab work done, but it's definitely more of a hassle.