I was thinking of a dampening field too, but the point about rechargeable batteries is valid. They would be long dead and only an influx of electricity would bring them back to life. Thus, it would seem as though the pendant with the flash drive thingie in it is a kind of power source that charges every battery around it within a certain radius. The charge is temporary and in effect only as long as the pendant is active. The batteries don't get charged and retain their charge for awhile after the pendant goes dormant, which means it's actively providing power but only enough to run electronic devices while it's on.
The fact that there is no known technology like this in the world, and the strange design of the pendant itself, says "extraterrestrial" to me. Or some crazy genius we haven't met yet. One or the other. If it was purely malevolent in design and purpose, then those people would have imposed their will by now.
Based on the flashbacks we saw, neither Monroe nor Miles knows anything about it.