There is a web site for National Trails Day events in Arizona.  It's at: http://www.azstateparks.com/partnerships/trails/participate.html  Below is what they have to say about Acker park.  They don't specifically mention Caching, but they do mention a search and rescue organization.  Could there be a connection?  :-)

National Trails Day
Sponsor: Yavapai Trails Association
Description:. Booths of various outdoor organizations, historic encampment, orienteering/group demonstrations, search and rescue organization, Forest Service representation and a nature hike. Free food and drink will be provided. Raffle for hiking white elephants and gear giveaways.
Date/Time: Saturday, June 7th, 9:00 a.m. – 4:00 p.m.
Locations/Directions: I-17 to Hwy 69 (Prescott exit), take Hwy 69 for 30 miles. Stay in middle lane to downtown Prescott – the name of the street becomes Gurley Street. Take a left on Virginia Ave. to end of the street to Acker Park, parking is available.
Event Contact:. Angie Finn, angie1441@aol.com, (928) 778-9666