Sounds like it might have been a heavily concreted (or stoned) area, since you never described the facility. At my last, hopefully not my last, I could pull AS sounding like doomsday on first counting. They went to watch in 12, to nothing in 24, and bkg in 36! We ran the final, record, count at 48!
Other sites with environmental AS we ran the last count at 14 days!