By Thomas Kee, guest columnist

The flagship exchange-traded fund for the S&P 500, SPDR 500 (SPY), has just made headlines by surpassing the $100 billion market cap mark. The move was hailed as yet another sign that the ETF industry has come of age.

But like a Time Magazine cover story, this headline may be a contrarian indicator for the market. 

Last Friday, $1.31 billion in new money entered this ETF, according to Index Universe, suggesting that many investors are comfortable buying the market again. This was a very large inflow, and raised some eyebrows. Who is doing all the buying and why? My question is, should prudent investors be selling when everyone else is buying?

The rule of prudent investing is to buy when everyone else is selling and sell when everyone else is buying. According to my observations, investors are not only buying, but they are unusually complacent about risk. This is dangerous on many levels. I'm especially concerned that a reversion to "Buy and Hold" strategies seems to be taking place yet again.

This seems to happen at least once a year. The market improves for a few months and investors stop respecting the risks. They shift their money into equities after the easy money has already been made. Then they start sitting on their hands while hoping for the best.

According to my observations, that is exactly what has been happening lately. Those investors who were too afraid to buy when everyone else was selling (in the summer) are now chasing the market higher, and buying at what I consider to be a relative peak. 

Instead of squatting on an ETF like SPY, my recommendation is that investors adopt a more proactive strategy. If the S&P 500 is your focus, use ProShares UltraShort S&P 500 (SDS) (the double short ETF for the S&P 500) and ProShares Ultra S&P 500 (SSO) (the double long ETF for the S&P 500) to accomplish this goal. Buy SSO when everyone else is selling, sell it when everyone else is buying. Then consider buying SDS when everyone is buying, sell it when everyone is selling. Repeat this simple process every year. 

That strategy doesn't make sense for you. But consider yourself warned: The inflows of new money into SPY and the headlines that make everyone feel warm and fuzzy may turn out to be a contrarian indicator. It surely is a sign that "everyone" is buying and I consider it a sell signal. 

Thomas Kee is the president and CEO of Stock Traders Daily, the founder of The Investment Rate and the author of "Buy and Hold is Dead."