We know Sandburg isn't all that crazy about rhyme scheme and all the devices that go along with them. Sure you may see a few in some of his poems, but they're not the prevailing elements. However, there are a few subtle sound patterns that we can hear in "Fog."
For example, line 2 has those "little cat feet" that sound the same way they look. The repetition of that T sound in those three words is not only an example of consonance but also has a staccato (short and choppy) sound that makes us think of tiny kitty paws. So we may have some onomatopoeia going on in that phrase that mimics the creeping pitter-pattering sound of a kitty walking.
Line 6 also sounds the way it looks and feels too: "and then moves on." The casual way the speaker ends the poem reflects the nonchalant way a cat (and fog) might "move on." There's no rhyme or reason (for people other than meteorologists) as to why it chooses to linger at some points and then suddenly move elsewhere. The cat-fog is mysterious, after all, and the sound of this poem helps to get that point across.