It could be worse
"... we measured more than 100V AC between the earth pins of sockets on each side of the hall ..."
It can be a lot worse than that. I was installing networks at a US military base back in the 1980s. The building in question was originally built in the first decade of the 20th century. The other half of it was added during WW2. The voltage difference between earths/grounds on opposite walls of the same flippin' room were over 400VAC. They weren't even fed from the same substation. And God help you if you didn't have a voltage isolation unit in your mains supply when a battleship or aircraft carrier switched between on-board and land power.