Most investors/readers know about the stream of U.S. economic statistics originating from the U.S. Commerce and Labor Departments, and from other Washington agencies, that form the basis for 'taking the pulse' of the economy.

But more experienced investors know about that group of 'unofficial statistics' that fill-in the economic landscape and frequently provide clues regarding future economic activity that the others do not. In this category, you'll find mall traffic levels, those infamous corrugated box orders, and package deliveries, as metrics of significance.

And another metric worth keeping an eye on, in the interpretation of stock exchange specialists? The demand and prices for fine art.

Fine art, antiques, and collectibles are the aesthetic knick-knacks of the gentry. Or as one New York Stock Exchange (NYSE: NYX) specialist put it, "A lot of the other metrics measure how the little guy is doing. Art demand measures how the big guys are doing."

The significance? "When the little guy is pulling back, that's a concern. But when the big guys are pulling back, now that's a problem," he said.

Art demand slowdown telegraphing global slump?

Moreover, a problem may be surfacing with the 'big guys.' Sotheby's, the world's largest, publicly-traded auction house has dropped about 20% in the past week on concern the global art market may be slowing, Bloomberg News reported. Sotheby's (NYSE: BID) shares declined 39 cents to $22.64 in Friday afternoon trading.

Sotheby's contemporary art auction on Wednesday was not a confidence builder. The sale totaled $10.4 million, well below the $14 million high estimate, will only 69% of lots selling, Bloomberg News reported. In comparison, during a similar sale a year ago, 81% of lots were purchased.