Hypoglycemia: Given the patient's history of insulin-dependent diabetes mellitus type 2, hypoglycemia is a common cause of lightheadedness. Even though orthostatic vitals are negative, hypoglycemia can cause symptoms without significant changes in blood pressure.

Dehydration: Elderly individuals, especially those with diabetes and hypertension, are prone to dehydration due to various factors like decreased thirst sensation, medication side effects (e.g., diuretics), and increased urine output. Dehydration can lead to lightheadedness.
Anemia: Common in the elderly and those with chronic diseases like diabetes, anemia can cause lightheadedness due to reduced oxygen delivery to tissues.
Cardiac Causes: Conditions such as heart failure, coronary artery disease, or arrhythmias can lead to lightheadedness. The patient's history of hypertension increases the risk of these conditions.

Stroke or Transient Ischemic Attack (TIA): Although less common, a stroke or TIA can present with lightheadedness, especially in the elderly. Given the potential for severe consequences, it's crucial not to miss these diagnoses.
Severe Hypertension: While orthostatic hypotension is ruled out, severely elevated blood pressure can cause symptoms like lightheadedness due to hypertensive encephalopathy or cerebral edema.
Cardiac Arrhythmias: Certain arrhythmias, such as atrial fibrillation or complete heart block, can lead to lightheadedness and require prompt diagnosis and treatment.

Vestibular Disorders: Conditions affecting the inner ear, like benign paroxysmal positional vertigo (BPPV) or Meniere's disease, can cause lightheadedness but are less likely in this context.
Neurodegenerative Diseases: Rarely, diseases like multiple system atrophy or Parkinson's disease can present with lightheadedness due to autonomic dysfunction.
Endocrine Disorders: Other endocrine issues, such as adrenal insufficiency or thyroid disorders, can cause lightheadedness but are less common and would typically be associated with other symptoms.