Stock investors might need to look out below. An esoteric financial markets indicator just registered a warning signal that historically augured in double-digit falls in the S&P 500, the index tracked by the SPDR S&P 500 (SPY) exchange-traded fund. The metric in question is the Marshallian K., which measures the liquidity in the economy. Recently, the liquidity started contracting... “[T]he Marshallian K. now shows liquidity not only deteriorating but actually contracting,” states a recent report from financial company Leuthold Group. That conclusion might surprise many people who look at the Federal Reserve’s money-printing program, which started during the financial crisis...