The Sensex at the Bombay Stock Exchange dropped a marginal 18 points, or 0.1 per cent, to close at 19,427 on the last trading session of 2012, on investor caution, amid weak global markets, today. The Nifty at the National Stock Exchange fell 3 points, to 5,905.But in 2012, the Sensex rose a good 26 percent, to post its biggest yearly gain since 2009.