Originally Posted By: dougwalkabout
But how many people in 10,000 know that anything-in-threes is a distress signal?

I would probably go for three groups of threes. Arranged like this:

Code:
...     _ _ _     ...


Many people who do not understand the meaning of three's as an emergency signal will probably recognize this specific one. Easy enough to do the above with a whistle, a bit harder with a mirror (unless you have a very specific target you are aiming at), even harder with firearm shots. I guess you could do it using a combo of a .22 pistol and a .308 rifle, utilizing their very different sound signatures, or possibly by using different spacing between shots to indicate short vs. long.