Tracks down my close-up glasses and magnifying glass...... there is one of these copper coloured bubble/indents on a facet of mine.
Sorry, not swapping, 'cos I've got a faceted inside out! I have seen these flat bottles turn up on ebay occasionally, I don't think they're terribly rare.
Hmmm, sprinkler system... and who would pay for it?
Also in a fire, you're talking hot, then adding cold water, I think there might be explosions!