Art Cashin is concerned bond buying signals the market thinks the U.S. economy is faltering and investors need to be "very careful about that."
Cashin, UBS' director of floor operations at the NYSE, told CNBC's Bob Pisani that "the performance of the (U.S.) 10-year (note) is almost miraculous."
While he thinks short covering is responsible for some of the gains, "I have to tell you, this is moving yields enough that it almost hints that the economy may be sputtering a little bit."
With stocks near all-time highs, a slowing economy could deprive the market of the fuel it needs to go even higher.
As for Thursday's technical levels, Cashin recommends keep an eye on 4000 for the Nasdaq Composite, which "remains one of the weak links."
If the index drops below that level "it could bring in accelerated selling."
He also thinks people should keep an eye on the area around 1848-1851 on the stock index.
—By CNBC's Alex Crippen. Follow him on Twitter @alexcrippen.