In Star Trek: Discovery, during the season's opening episode, they are stranded on the planet and have no way to communicate to the ship due to storms. So the captain walks in a pattern to make the Starfleet insignia in the sand visible from space. This seems impossible since even the Great Wall of China is not actually visible from space without aid, and they didn't even know to be looking for any visual sign (as far as we know). Outside of using sensors, unless they are searching for the symbol, they would never even know to look for.
How are they found?