It's a good idea to own a few basic tools, including a handheld drill and maybe a hacksaw, even if you live in a small flat.
Although in this case, depending on how long the thermos had been shut, I'm not sure whether attempting to relieve the pressure would have been the smartest move.
Now I'm genuinely wondering what I would have done...
Like dsfsdfd said, leaving it on the balcony under a heavy blanket seems like the best option, though my first thought was just throwing it down the street sewers.
Yeah, thinking about it some more, "wondering what I would have done" quickly turned into "wondering whether I'd still have hands or a face right now." Scary stuff.
Drilling a hole is not a good option. If there is significant pressure inside then the hole will begin the explosion, not release the pressure. In an ideal world, the best option is to puncture it from far away, which is difficult out in a city. In the country, you might find someone to shoot the thermos with a small rifle.
Although in this case, depending on how long the thermos had been shut, I'm not sure whether attempting to relieve the pressure would have been the smartest move.
Now I'm genuinely wondering what I would have done...