My only other guess is powdery mildew, which in peppers apparently only produces spores on the underside of leaves and does not always cause discoloration on the top surface of the leaf, at least until the disease is well advanced.
http://www.agf.gov.bc.ca/cropprot/peppermildew.htm
http://vegetablemdonline.ppath.corne...eryPowdery.htm