Got the clue.  It was under the bridge.  It was a sheet of paper
with a 5wx8h grid of images on it.  It took us a while to figure
it out, but I can't believe myself for letting it do so, as it
should have been obvious to my amateur radio self...  It was
images representing things from the international phonetic
alphabet.  There were images of Oscars (the award, the character
from Odd Couble, the grouch), Mikes (Stipe, Meyers, Jordan,
etc.), India scenes, Tangoes, Uniformed people, etc.

Anywho, we figured out that it spelled:

HUMPHREYSMONUMENTATENDOFMAINSTINRIOVISTA

I.e.:

Humphreys Monument at end of Main st in Rio Vista