Wasn't it Paul Bramscher who wrote:

So it may be that larger clusters of intelligence (large cities) will be 
one of the earlier potentially remote-sensible artifacts of 
intelligence.  For example, cities might be approached as (often, in 
today's modern sense) having some signature (chemical, albedo, 
geometric, etc.) which make them often stand out.

We may never be able to pick out an alien's brain waves, but we just 
might detect a city on an earthlike planet in the next century or two...

That's a rather anthropomorphic concept of aliens. I'd guess that most
intelligent civilisations wouldn't build cities.

-- 
Mike Williams
Gentleman of Leisure