At the kind of forces this paper talks about it doesn't take much charge buildup for some nearby electric field to contribute force via Coulomb attraction. Any kind of grease stain or oxide patch can do this.
This is fascinating. How would you go about determining it as a source in general? I'm thinking of some slight force being at an angle to generated the effect, but that's pretty much principle of force superposition in action. The paper itself is way over my head, and will likely stay that way for quite a few years, but I'm curious how experimental specialists deal with grounding and surface charge 'residue'.
Seriously, I want all the details you can give me here.