Lost Frost plugs

I'd definitely pull it out and check ALL of them, I also agree with pressurizing it to make sure there are no leaks. I have also seen engines crack in the valley under the intake but to pressurize that, you would need to build plates to block off the water ports in the heads as well.....

Although, if there was only just a little bit of water in the bottom of the block, it may have just froze and popped those 2..... bummer though.....