Just saw these HORs on a siren GPS site...don't know if they still test sirens in Scottsdale, AZ, but that appears where these are located:
http://www.waymarking.com/waymarks/deta ... 6fbdee1f37
http://www.waymarking.com/waymarks/deta ... 53b165495b
The one at Navajo seems *really* close to the ground!