The Sensex at the Bombay Stock Exchange gained 104 points, to 18,370 in opening trade, Monday morning, on continued buying by investors. But the Sensex later surrendered all its initial gains, to stand 5 points in negative territory, at 18,261 in afternoon deals, amid volatile trade, a short while ago. The 30-share Sensex had gained over 400 points in the last two trading sessions. Other stock markets in Japan, China, Singapore, Hong Kong, Indonesia and South Korea were trading mixed, on Monday.