ATP is the chemical that the mitochondria in our cells turn all that sugar into for cells to burn in all sorts of energy-consuming processes. That means it's a pretty good indicator of the existence of life on counter-tops and refrigerator handles.
A recent study by Coverall Health-Based Cleaning System looking for ATP residue in a child care center found that ATP levels were higher in the morning, after a cleaning crew had been through the place than the night before, after staff and kids had gone home. They attributed it to "cross-contamination, usually caused by antiquated cleaning methodology and technology." I'll bet they concluded their technology wouldn't do that.
I've sent for a copy of their full report and will let you know what I think after I've read it.