With the US stock market at a record high and daily stock gyrations near multi-decade lows, some investors have raised concerns about the lack of fear in the market, but US equity options market data suggests investors are far from complacent.

Positioning in options on S&P 500 index and CBOE Volatility Index shows investors have been gradually adding to hedges over the last few months.

“We didn't see it on our desk and no one seems to care much about hedging but somehow it's happening,” said Jim Strugger, derivatives strategist MKM Partners in New York.

“It's sort of under the surface, more like stealth hedging,” he said.

The S&P 500 index .SPX has climbed 16 percent this year and is on pace for its eighth straight month of gains, the longest such streak since just before the 2007-2009 financial crisis.

The CBOE Volatility Index, better known as the VIX and the most widely followed barometer of expected near-term stock market volatility, closed at a record low earlier this month.

Some investors warn that heightened reliance on strategies that profit from continued calm in stocks, and months of frustration over hedges that have gone to waste while the market powered on, have left the market extremely vulnerable to a shock.

Boom-time complacency should top the worry list for investors, according to participants in the recent Reuters Global Investment 2018 Outlook Summit in New York.

The options market, however, suggests that investors are not as vulnerable to a sell-off in stocks as the anemic level of the VIX would suggest, analysts said.

For instance, for the S&P 500 index options, there are 2.1 puts open for each open call contract, close to the most defensive this measure has been over the last five years, according to options analytics firm Trade Alert data.

An index call option gives the holder the right to buy the value of an underlying index at a fixed level in the future. A put conveys the opposite right and is usually used to protect against declines in the index.