Gilbert's Syndrome: A benign condition characterized by a mild increase in unconjugated bilirubin, often exacerbated by stress, fasting, or illness. It's common and can present at any age, including in the elderly, especially if previously undiagnosed.

Drug-Induced Hyperbilirubinemia: Many drugs can cause an elevation in bilirubin levels, either by increasing bilirubin production or by affecting the liver's ability to conjugate or secrete bilirubin. Given the high likelihood of polypharmacy in elderly patients, this is a plausible explanation.
Hemolysis: Mild hemolysis can lead to elevated bilirubin levels. Conditions such as autoimmune hemolytic anemia or microangiopathic hemolytic anemia could be considered, especially if there are other signs of hemolysis like anemia or elevated lactate dehydrogenase (LDH).
Chronic Liver Disease: Conditions like chronic hepatitis or cirrhosis can lead to elevated bilirubin levels. However, these conditions often have other associated symptoms and findings.

Hepatocellular Carcinoma: Although less common, hepatocellular carcinoma can present with mildly elevated bilirubin among other symptoms. Given its severity, it's crucial not to miss this diagnosis.
Cholangiocarcinoma: A malignancy of the bile duct that can cause obstructive jaundice, which might initially present with mildly elevated bilirubin.
Acute Viral Hepatitis: Though more commonly associated with significant elevations in liver enzymes and bilirubin, mild cases can occur, especially in the elderly, who might not mount as vigorous an immune response.

Dubin-Johnson Syndrome: A rare genetic disorder characterized by chronic jaundice due to an inability to secrete conjugated bilirubin into the bile. It's rare and usually diagnosed earlier in life but could be considered in the differential if other causes are ruled out.
Rotor Syndrome: Similar to Dubin-Johnson Syndrome, it's a benign condition causing mild elevation of conjugated bilirubin, without liver injury. It's very rare and typically presents in childhood or young adulthood.
Zellweger Spectrum Disorders: A group of rare, genetic disorders affecting the liver and other parts of the body, which can lead to elevated bilirubin among many other symptoms. These are typically diagnosed in infancy or early childhood but could be considered in undiagnosed adults with multiple system involvement.