I have tested earth paths around my workbench, which is where I am using the scope. I have not tested the earth path of the actual outlets in the wall since that would need me to turn the power off, but other than that I have not found any problems.
Correct me if I am wrong, but I don't think this stray current issue actually poses any risk to me or my electronics, so I don't think it is worth chasing any more. If I had to guess, a mixture of long usb cables and a sub-optimal set of power strips feeding my PC and workbench make this seem much worse than it normally would be.