(Read more: Warren Buffett on why a bull market is like sex)
The index most recently traded around 132, which is above its 50-day moving average and well above the 200-day moving average.
Anything above 100 is considered a sign that traders are nervous, though it usually takes until about the 130 range before anyone starts noticing. The measure spent most of January below that level but recently breached it.
Even market bulls believe there are considerable signs that fear is creeping back into the market after a strong February that has seen the S&P 500 gain nearly 3.5 percent in price and come just shy of breakeven for the year.
(Read more: JPM's Lee: This is not a mature bull market)
In his morning note Thursday, Jeffrey Saut, chief investment strategist at Raymond James, said:
My short-term internal energy indicators are totally out of energy, the equity markets are pretty fatigued, the CBOE Skew Index has registered a reading whereby the option traders are expecting a downside 'black swan' event over the next 30 days, optimism is very tilted to the upside, and every overbought indicator I monitor is WAY overbought. As for me, I am confused awaiting the equity markets' trading resolution as to their near-term direction."
Saut remains convinced that this is a "new secular bull market" but is concerned over the turmoil in Ukraine as well as a general climate of European deflation.
(Read more: Escalating tensions in Ukraine roil global markets)
Similarly, Michael Hartnett, chief investment strategist at Bank of America Merrill Lynch, is long-term bullish but sees ramifications of what he calls the "New Paranormal" possibly exacting some damage in areas of the market.
Hartnett explained in a note to clients:
As noted last year...2014 could be a year of speculative excess rather than one of macro, rate and asset allocation normalization. It remains hard to believe that an era of unprecedented intervention in financial markets by central banks will not threaten a parabolic overshoot in some asset prices.
Potential candidates: US biotech, internet stocks, floating rate debt.
However, Hartnett believes the long-term picture is supported by easy monetary policy that will keep interest rates low as well as a belief that the soft patch in economic data that has greeted 2014 will be reversed:
Today rates are zero and likely to stay that way for the foreseeable future. And capital flight from emerging markets will add fuel to the fire as well as feed the strong bull markets in prime real estate in the West as well as high quality, high growth Best of Breed stocks.
—By CNBC's Jeff Cox. Follow him on Twitter