I would like to propose an alternative to those dismissing the advent of possible complex electrochemistry behind the phenomenon and asserting instead perhaps just poor planning and/or design decisions.
In this regard, an interesting 2011 article, 'An Fe3O4–FeO–Fe@C composite and its application as anode for lithium-ion battery', discussed here, to quote:
X-ray diffraction, Raman spectrometry, scanning electron microscopy with energy dispersive spectroscopy, transmission electron microscopy and galvanostatic cell cycling have been used to characterize the structure and electrochemical performance of the as-prepared Fe3O4-FeO-Fe@C composite. The electrode shows a stable and reversible capacity of over 600mAhg−1 at a current of 53mAg−1 for up to 60 cycles, which could be ascribed to the unique double-layer core-shell and embedded structures.
Also, per another reference discussing the same electrode, to quote:
Here, we report the synthesis, characterization and evaluation of the electrochemical properties of pristine Fe3O4 nanoparticles synthesized from a base catalyzed method mechanically mixed with electrospun Polyacrylonitrile (PAN) derived carbon fibers as simple composite material. The Fe3O4/C electrodes are characterized by galvanostatic cycling experiments using currents as high as 2000 mA g−1, revealing high capacity values and capacity retention, together with a very good capacity recovery during the rate capability experiment.
The picture appears to display a red pigment, possibly Fe3O4 created at elevated temperatures. Other bars, resemble Fe/FeO.
So speculatively in the current case, an alternate proposition that if the iron metal fence was exposed to say a brush fire, heating the iron and also depositing fine carbon, this may have resulted in a series of Fe-FeO@C and Fe3O4@C electrodes naturally developing over time (aka, yes, some possible complex electrochemistry).