Corporate insiders are selling much more stock than they're buying, and some see that as a warning sign for the rally.
"Insider sentiment remains bearish," with the NYSE-ASE eight-week sell-buy ratio at about 4, according to the Vickers Weekly Insider report published on Monday by Argus Research.
When the ratio of sells to buys is greater than 2.5, it "suggests a weakening market," according to the report.
"It's an interesting data point," said Gina Sanchez, CEO of Chantico Global. "People do sell for different reasons, but when you get some sort of mass corporate selling where everybody's sort of doing the same thing, you have to ask yourself: Why aren't people investing with their company or keeping their money with their company?" she said Tuesday on CNBC's "Trading Nation."
The high level of insider selling "supports some of the other pieces of data that tell us that the rally has been breaking down for a few months now," Sanchez added. She further argued that seasonal factors, high valuations and optimistic earnings numbers make the market vulnerable.
On Wednesday morning, the S&P 500 fell by more than 1 percent.
To be sure, not everyone sees meaning in the insider transactions data.
"If anything, I would say the warning should be if you're short the market," since the ratio "has been on a 'sell signal' through the last year now," Oppenheimer technical analyst Ari Wald said Tuesday on "Trading Nation."
In general, Wald says that investors are looking for reasons to be worried, since "we're still scarred by these memories of 2008. And it's created what we're calling this 'fear of heights,' where [during] every market rally, the assumption is that it's going to be followed by the next market collapse."
These concerns aside, the market's breadth and its cyclical leadership suggests to Wald that stocks are in "the early or middle innings of a new advance."
"We think the market continues to go higher, and this insider selling ratio is not something to be worried about," Wald said.
According to the Argus report, the stocks most heavily sold by insiders in the past 90 days include Zillow and Wayfair. Meanwhile, the names most heavily bought include FleetCor Technologies and Kite Pharma.