i doubt they are "hearing an explosion" to be said for some military training, etc.
these things are doing physical movement on items.
if anything, i would think sonic boom. and the military always denies until they are caught.
gas line? no someone would smell it.
plates? no cause they rumble.. if a plate was causing the boom, then you would think the plate was dropping 5 sudden feet or something.. cause these are sudden booms, not rumblings. and plates cause earthquakes and tidal waves, not a boom and hit your stuff down.
bomb?
i dont know much about bombs, but something causing that large of an area to be effected would have some kind of mark left on the area it was set off.
an air wave boom even causes a disruption in the plants, ground, etc. so if it was a bomb, there would be tracks left.
no idea