Freze Plug

I gotta agree with 67... They are sand core plugs and just happen to pop out when the block get's under pressure during a freeze. If you think otherwise, ask yourself how the heads crack on the exh flange side when they freeze but the core plugs in the heads don't pop out...
To the OP... I'd be very careful looking the block over when it comes out, and if possible, let it thaw, then use a radiator pressure testor to pump up the cooling system to 15psi or more. If the pressure drops and you cant see a leak like a hose lose or something, there's a very, very good chance the block has a crack. I have seena valley crack, but most of the time the valley cracks and main cracks are not from freezing but from use. The sides of the block crack from freezing, and the heads along the bottom edges near the head bolt flanges. Least that's my experience.
Also, if you run water and the car is sitting for more than a week in climates that have ever experienced a cold snap... Drain the block. It's as simple as removing the screw in drains in the sides of the block and replacing with radiator petcocks.