Dec. 4
http://www.oregonlive.com/portland/index.ssf/2010/12/multnomah_county_sheriff_dan_s_2.html
As the criminal investigation proceeds, so does a search-and-rescue operation. Its braincenter is located in a small room down the hall from the "War Room." Marked-up maps of the West Hills and Sauvie Island cover a conference table; maps of Sauvie Island are taped to the walls, a map plotting cell phone towers around the school and Sauvie Island is affixed to an easel, and maps from the earliest searches for the missing boy are inventoried in cylindrical bins.
Searchers from three states, including planes, search dogs, all-terrain vehicles, horse and walking teams, have combed heavily-wooded ravines, thick grass fields and hilly rough terrain around Skyline School, the Horman home and Sauvie Island, the largest river island in the country.
In recent months, searches at the island resulted from cell phone tower pings analyzed, viable leads or statistical probabilities from studies of other missing child cases, officials said.
So this confirms it's not JUST cell phone pings that are causing them to search SI.