Nervous S&P 500 traders bet the market's best days are behind it

Even strategists lifting year-end targets warn of bumpy ride

The relentless rally in US equities has driven bears almost into extinction. But to say bullishness is ubiquitous would be a stretch.

From options trading to stock preferences to the direction of retail money flows, signs of trepidation are budding in a market where $27 trillion has been added to equity values in a little over a year.

A once-abandoned option trade that profits in a go-nowhere market is back in vogue, signaling caution. Funds that employ the strategy are receiving money at the fastest pace in nine years. And while reopening stocks roared ahead Friday thanks to positive jobs