Sectional Chart Question

A pilot recently asked me a question, that I cannot find the answer to.

On a sectional chart, around cities, are yellow areas.
What are they?
Where on the legend can this be found?

My answer was "congested areas of a city or town" which I though sounded
official
enough to work. Ah-Ha, he says, "Show me."

The specific area in question is just south of Sunriver, on the Klamath
Falls sectional. (DSD 175 @ about 25nm) I've flown over it, and it is a
combination of golf courses.

Over beer last night, one fellow suggested that it is marked that way
for "National Security Reasons".
Wink, Wink, nudge, nudge, "You are not supposed to ask.", "Why do you think
they left it off the legend."

Am I just blind? Is the print too small? Can anyone else find it on
the legend?

Al Gerharter