I agree that the chance of the coolant catching on fire inside one of Tesla's batteries is probably extremely small, but apparently using coolant in sprinkler systems as anti-freeze can in fact be flammable and cause problems if the concentration is too high. According to this article there was in incident where someone was killed when the sprinkler system was set off by a kitchen fire.And i'll just reiterate my fun fact from earlier: Similar coolant to that used in the Model S is used in fire sprinkler systems around the world that are installed in areas that can get below freezing. I've worked in this field in the past, and know quite a bit about it. When a fire sprinkler on one of these wet subzero systems goes off due to a fire popping the fuse in the sprinkler head, the first thing dousing the fire? Coolant. Hundreds of gallons of it can rain down on the fire as a spray/mist before water follows it. If this stuff could actually cause or worsen a fire... um, well, it wouldn't be used in actual fire protection systems like this. Common sense? It doesn't catch on fire. It doesn't cause fires.
#107 - The Use of Antifreeze in Fire Protection Systems
Looks like NFPA will not allow using propylene glycol or glycerin in sprinkler systems starting in 2022. There is also apparently a UL-Listed ant-freeze that you can use safely.