What's a leading indicator? It's a data series that can be used to reliably predict the future movement of another. But informal data and anecdotes can be just as sensible and accurate as formal leading indicators.
For example, to measure the strength of current housing markets, people are noting the year-to-year increase in inventory, the relative decrease in price growth or a decrese in prices, etc.
But the most powerful indicator might very well be the return of highly visible clusters of lockboxes outside of condominium buildings. (w/ pic goodness).