He came through the nether. Built a staircase up to a hidden - not visible form any direction - nether portal, and dug into a sealed nether wart farm. No other digging, just straight to his targets. He went out throgh the portal at ~7250:100 into a rail-station, pierced the wall exactly where a useless chest stood and got to the chests at the two other stations 0,5k away, destryoing rails on his way. Maybe he fond on of the chests by accident and guessed the other ones. Maybe he went through the portal into nether and not the other way around. But the straightnes of his piercing of the chests and farm says everything.
This is a warning to everyone next to the location. Maybe someone has seen something suspicious to find out who it was...