A 62-year-old female presents to her general physician's office complaining of intermittent, non-exertional chest discomfort over the past month. She describes the discomfort as a pressure-like sensation, occasionally radiating to her left arm, but not consistently reproducible with activity. Her medical history includes well-controlled essential hypertension and type 2 diabetes mellitus managed with oral agents. She denies any prior myocardial infarction or revascularization. Physical examination reveals a blood pressure of 138/82 mmHg, pulse 72/min, and respiratory rate 16/min. Cardiopulmonary auscultation is unremarkable. Peripheral pulses are symmetric. Her baseline electrocardiogram, available from a routine check-up two years ago, shows a stable left bundle branch block. A repeat ECG today is unchanged. Considering these findings, what is the most likely underlying cardiac etiology to rule out, and what is the most appropriate initial non-invasive diagnostic strategy?