After studying the clues, I've determined that the poor, innocent puppy was lured onto the treacherous, snow dusted swimming pool ice by Callie, the conniving calico cat. The cat exited through the railing on the far side of the pool. The puppy, however, has been eating well and no longer fits through such narrow gaps.
Fortunately for the puppy, aka Bebe, it's been wickedly cold this week. The ice didn't crack. Cat chasing, however, must have been a challenge. So was the big leap from the ice back onto the deck.
A side note: we're trying out a new type of pool cover this winter. We opted for a mesh-type that would act as a strainer and allow the rainwater to filter through but catch the leaves. (Always an issue when surrounded by oaks that seem reluctant to part with their leaves in a timely manner.) Theoretically, this new cover should make spring pool cleanup a bit easier. Those light streaks in the cover are supposed to be reflective strips that melt snow and ice. Hope the spring cleanup theory proves more effective than the melting theory.