90% of the year, this parking lot is used for runners, walkers, bikers, etc accessing the trail along Lady Bird Lake and some overflow from Zilker Park. The other 10% of the year, this parking lot is closed for big events at Zilker Park like ACL; however, the trail is always open. This cache is hidden along the trail, so 90% of the year you shouldn't have a problem, and the other 10% you might need to walk to it...
Anyway. This is a VERY public area depending on the time of day. Check out the map of the trail and you'll be close. A keen eye will see what's different. If you walk across the pedestrian bridge, there are drinking fountains and bathrooms and Austin High School. If you head east, you'll basically stay between the river/lake and Zilker Park.
*NOTE - last time I checked, there was an AL stage near this location!