We checked with Ford spokespeople Alan Hall and Jennifer Moore (who specifically work on sustainability communications) and both told us that there is no work ongoing on an electric Fiesta. Since the plug appears to be a standard 110V three prong unit, this is almost certainly just a block heater. While a heater wouldn't be needed in the sunny summer locale in Mexico, this prototype has likely seen service in cold weather regions at other times.
As for the reported silence, Ford has said almost all of its cars will have automatic start-stop capability within the next few years and the Fiesta is expected to be one of the first so equipped. Thanks to Francisco for the tip!