Iron Deficiency Anemia: This is the most likely diagnosis given the patient's history of low MCV, which is a hallmark of microcytic anemia, and her treatment with oral iron pills. The lack of identifiable gastrointestinal bleeding despite thorough investigations suggests that the iron deficiency might be due to other causes such as menstrual losses, dietary deficiencies, or malabsorption.

Thalassemia Trait: Given the patient's Greek origin, thalassemia trait is a plausible diagnosis. Thalassemia can cause microcytic anemia with a low MCV, and it's common in Mediterranean populations. The absence of significant anemia or other symptoms might suggest a trait rather than a major form of thalassemia.
Anemia of Chronic Disease: Although the patient does not have an overt chronic disease, conditions like chronic infections, autoimmune diseases, or even undiagnosed malignancies can lead to anemia of chronic disease, which can sometimes present with a low MCV.

Celiac Disease: This condition can lead to iron deficiency anemia due to malabsorption and can be asymptomatic or present with non-specific symptoms. It's crucial to consider celiac disease as it requires a specific dietary intervention.
Gastric Cancer or Other Malignancies: Although the patient has undergone negative gastrointestinal endoscopies, the possibility of a malignancy cannot be entirely ruled out, especially if there are other risk factors or symptoms. Gastric cancer, for instance, can cause iron deficiency anemia through chronic blood loss.
Chronic Kidney Disease: Anemia is a common complication of chronic kidney disease, and while it typically presents as normocytic, in some cases, especially with concomitant iron deficiency, it can be microcytic.

Sideroblastic Anemia: This is a group of disorders characterized by the presence of ringed sideroblasts in the bone marrow. It can be congenital or acquired and may present with microcytic anemia.
Lead Poisoning: Lead interferes with heme synthesis, leading to microcytic anemia. Given the patient's occupation as a computer engineer, this might be less likely unless there's an unusual exposure history.
Other Rare Genetic Disorders: Such as congenital sideroblastic anemia, or other rare causes of microcytic anemia, which would typically require specific diagnostic tests for identification.