Leaky radiators are a big problem. My 1971 Camero used to leak on my garage floor every time I pulled into the garage.
Had the car for many years and was always putting coolant in it. Never had a fire, even when it backfired out the carb, which was often.
The NFPA rates flammability on a scale 0-4
0 = minimal
1 = slight
2 = moderate
3 = serious
4 = severe
They rate pure e-glycol a 1
It certainly will ignite, I just did it awhile back with some newspaper as a wicking material.
I also read that heating pure e-glycol can produce vapors that can flash back to a flame source, but even then no mention of explosion.
Here is a real life coolant car fire - still, no explosion.
http://www.croberts.com/coolant.htm
I also spoke with a Police friend of mine with 25 years tenure. He said garages actually blow up more than people think. He said it's almost always the same thing - improperly stored gasoline in a garage that has a gas water heater in it. I asked him about coolant fires, he said they are a common event along side the road when a car is boiling off it's coolant, but he never heard of one in a garage. So, in summary I am 100% convinced you can start a nasty fire with e-glycol, I am just not sure how the circumstances you describe in the over-night scenario could cause an explosion event violent enough to knock someone out.
You can even get massive explosions with nothing but dust from grains of wheat mixed with air.
I proposed a way for "drying out" the ethylene glycol sprayed out of the radiator several hours earlier when she pulled into the garage and ran into a work bench.
Remember e-glycol evaporates at a higher temperature than water, and freezes at a lower temperature than water. That's why it's added to the water.
If you spray the anti-freeze out through tiny holes in the radiator you create a great deal of mechanical separation of the solution that makes it much easier for the water and e-glycol to go their merry ways ~ the water into a condensate on cold metal automobile structures and parts ~ e-glycol into a cold spray drifting around through the unventilated garage.
Eventually you get a very nice e-glycol and air mixture.
I think ATF prefers an electrical detonation here hence the targeting of the lightbulbs. I think the heating coil on the catalytic converter is more likely since it tokes up to about 1300 degrees F.
Now you can start a fire most anywhere with that!
It's possible ATF never looked at the catalytic converter not realizing that BMW has had a modified device in use for many years.
Now, about when the cat starts heating up or the lights turn on? The second the doors open will turn on the lights. Still she managed to get into the car. That's where I rule out the lights. If there was going to be an electrical detonation it would have happened immediately ~ which is obviously not the case.