Erasing initial gains, the Sensex at the Bombay Stock Exchange dropped 26 points, or 0.2 percent, to 15,857, as investors adopted a cautious approach ahead of the third-quarer corporate results, and on subdued overseas markets, on thursday. The Nifty at the National Stock Exchange closed unchanged, at 4,750.Stock markets in Japan, China, Hong Kong, South Korea and Singapore again ended mixed.