I bet he heard a mole or something else under the snow. Our pasture is over run with moles and gophers right now and Stosh does the same thing- foot up, ear tilted to the ground, staring. At first I thought he was looking at a bug but then he pounced on the ground and started digging. He will actually follow them along and track their movement. But knowing Wolfie, he might actually be seeing them!!