We get the same thing around here even on frozen lakes with 2 or 3 feet of ice. If you get a large snow fall that puts a lot of weight on the ice there will be areas with enough cracks that water will seep up under the snow. We call them slush holes. The water seeping up mixes with the snow to make slush and it won't freeze for days even in below zero temps because the deep snow is unsulating it. They are usually discovered as you are driving your car or snowmobile across the lake. All of a sudden you hit what feels like a mud hole and you start sinking and slow down fast. Most times the ice is still intact underneath and I've never known anyone to fall through but you may get stuck in the slush hole and it can be scary. I've seen it where the slush was a foot deep. There are many of these on most lakes when we get a lot of snow and people drive through them all the time. Once they are discovered you avoid them. Personally I stay off the lakes with my car until there is at least 16 inches of ice and then only drive where others have already gone thus avoiding these hazards.

Gotta get back to fishin!