In reply to monkeys:
Oh, you're right; hot in a few seconds is certainly not right...
My guess would be that there was a latent fault in the weld (a weakness leading to a pinhole, perhaps), and this failed when you washed it, or whilst it was sitting in the cupboard. It may even have sucked water into the vacuum space, but I think you'd notice that by the sloshing noises...
The explanation was intended genuinely, BTW, even if, re-reading it, it does sound a bit sarcastic. I've experimented with making lightweight insulated cups out of nested drinks cans, with foam in between, and the main heat loss mechanism is exactly as described above, with the heat conducting up the inner, over the joint at the top, and down the side of the outer.