A friend and I are debating the real world usage of this technique:
http://phys.org/news/2014-04-smartphone-sensors-trackable-fingerprints.html
Basically by watching the measurements given by a specific piece of cell phone hardware researchers were able to identify unique phones by the profile of their hardware signals.
The argument is whether EMF from everyday sources would skew these profiles. I just don't think that the EMF from the earth or a powerline would have enough "weight" to even show up in the profile.
Best Answer
What these researchers seemed to be doing was accessing the data from within the device using an application. External EMF could absolutely induce spurious noise and change the signal. The thing is, for these devices to work, they have to be fairly immune to noise.
Also, their technique would likely involve a series of filters. Once they had the raw data, they would have to take it apart and look for the "fingerprint." That would have to exclude noise and anything common to all devices.