I've seen that exact same thing - 24 volts - we had one of the phases crap out and someone in the local area went and started up a genset and backfed the current into the grid without making the proper offlodading from their genset back into the line.
Not very likely .... a large load will just stall the prime mover on most generators, not let it run producing very low voltage. The most likely cause of low voltages like this is an open phase on the utility primary.
Don
