I used an ammo can in 2006, still there and in great condition and is the oldest cache in Noble County, Ohio! Many others in the area have come and gone in those 18 years.
My Garmin GPSMAP 65 is way more accurate than my Iphone, it will consistantly get me within 6 Feet, my cell phone around 20 feet at best. This is my experience, yours may differ.