The fuel dump valves on a 747 are from both wings,
V1.
Unless the aircraft had an air turnback, or a divert (divart, in Nihongo) for an emergency, then it was more likely the dreaded "contrails",
bp.
(Did you observe a cockpit window open, by any chance?? If you did, it MIGHT have been an emergency dump!!
)
Fuel dumping is generally only done in cases of overweight landing - and then the aircraft is allocated a specific area - usually over water.
Occasionally, excess fuel - due to over-fuelling, expansion, or the transfer of fuel from one tank to another - may result in fuel being spilled overboard, without the knowledge of the crew.