The hollow in question is the site of the Sockburn Park Pool. A remnant of the old pool complex remains, which helps to lend a spooky feel to the area. Somewhat to my surprise, the pool wasn't killed off by the quakes. It reached the end of its economic life in the early 2000s, and was (largely) demolished in 2007. According to a 2007 press release "The toddlers’ pool and adjacent barbecue area are being retained to create an area for families to enjoy picnics throughout summer". Sadly, the pool and bbq area are in a sorry state these days,
The cache is a large black pill bottle that fits snugly in a gap, about waist height.