This is hidden at near the White Tanks Cemetery. This cemetary is used to bury people from Maricopa County that die without a way to pay for a burial elsewhere. According to articles about the cemetery the graves are dug by inmates.
It is only open to go into the cementery on Thursdays but cache is hidden outside the fence. Thursday is also the day burials take place and I've been told there is a Nun that has been there nearly every Thursday for decades.
To read more about this interesting spot: http://cronkitezine.asu.edu/fall2007/whitetanks1.html