Antifreeze leak after cold spell

[Moriarty]

Went out to the garage a couple days ago and noticed a stream of antifreeze had leaked from underneath the 427.

Last week, we had temps as low as 18F amid a string of three days below freezing. The car is parked in a detached garage which is finished, but not heated. We went from a long Summer here in Seattle straight to Winter, and I have never gotten around to doing anything special to winterize - plus I usually plan to do some work on it over the colder months.

I haven't touched the coolant for a couple years other than to check levels (which have been fine). Didn't get much driving these past two summers.

Anyway, I use a 50/50 mix of green antifreeze. I've always expected that to be fine at much much colder temperatures than it ever gets here. It looks like this should be good to -10F. I have a hydrometer on order so I can test it.

It appears to have leaked from the bottom of the overflow reservoir, and also has some brown gunk - corrosion?

I popped the cap off, and the level is maybe 3/4" below the inner baffle. When I check, I'm usually looking to see if it's about level (which it typically is, maybe just a bit below? I don't remember).

I'm confused what happened here. The amount I discovered on the floor (maybe a pint's worth?) seems about in line with the current level in the reservoir, but if there's a leak there I was expecting it to be empty. This seems to have happened all at once (and has since stopped), not just a slow leak.

I expect frozen coolant within an engine block to be pretty catastrophic, but don't think it got cold enough for that. Does coolant separate while sitting ever? The engine hasn't run for several months.

What tests should I run come Spring when I'm ready to get things running again?