Stasis Dermatitis: This condition is common in elderly patients due to decreased mobility and venous insufficiency, leading to large, flat, red, and uninflamed spots on the lower legs.

Eczema (Nummular Dermatitis): Can present with large, flat, red patches, often on the lower legs, especially in elderly patients.
Psoriasis: Although typically more scaly, some forms of psoriasis can present with large, flat, red plaques.
Seborrheic Dermatitis: While more commonly found on the face and torso, it can occasionally appear on the legs, especially in the elderly.

Malignant Melanoma: Although less likely, any new or changing skin lesion, especially in an elderly patient, warrants consideration of melanoma due to its potential lethality.
Basal Cell Carcinoma or Squamous Cell Carcinoma: These skin cancers can sometimes present as flat, red lesions and are critical to diagnose early.
Vasculitis: Certain types of vasculitis can present with skin manifestations that might resemble large, flat, red spots, and missing this diagnosis could lead to significant morbidity.

Mycosis Fungoides (Cutaneous T-Cell Lymphoma): A rare form of skin lymphoma that can present with various skin manifestations, including large, flat, red patches.
Granuloma Annulare: A benign condition that can sometimes appear as large, flat, red rings or patches, though it's more common in younger individuals.
Necrobiosis Lipoidica: A rare condition often associated with diabetes, presenting with yellowish or reddish patches on the lower legs, which could be considered in the differential, especially if the patient has diabetes.