The overt message is the final line, "Somebody loves us all."
The question concerning this message is: How do we know someone loves us all? This could go through a number of tangents. One interpretation, spiritual, is that everyone is watched by a God, angel, or some benevolent being who cares for us certainly in the most dire (or dirtiest) situations. Another interpretation, and this one is probably closer to Bishop's intent, is that there are small, seemingly trivial clues that provide evidence that we are loved, or at least thought of. In other words, while love is abstract, there are physical traces of it if you stop and consider the often unnoticed ways people have helped you in your life.
The daily upkeep of the station and the little efforts like watering the plant (keeping life alive) show evidence of these traces of thoughtfulness. Just as there are physical traces of love/thoughtfulness, people sometimes add even more significance to these traits. The speaker does this by comparing the SO-SO-SO echo as a parallel to the traces of the mother's efforts.
These are "traces" because the mother is absent. We see the fruits of her effort but we don't see her in the act of performing these jobs. Calling her "somebody" gives her effort even more credence because she didn't stay around to get credit. The gift is all there is.
The filling station itself can be thought of as a microcosm (smaller example) of other families and the world at large. The example that came to mind with this poem and the idea of the "absent giver" is flowers in a cemetery. Cemeteries can seem like drab, melancholy, depressing places. Placing a flower on a grave is done to indicate the deceased person was thought of. This, and the mother's efforts in the poem, are clear examples of "it's the thought that counts."