How the SOS signal can get through if comms are going to be blocked is a question I leave for the interested viewer.

Maybe they went to the surface, sent it up through the atmosphere, and then went back inside. If the ship’s records say the SOS was last received from the mouth of this cave, it’s a pretty easy assumption. You could even send along,

SOS, SOS! We have wounded in a cave on [insert planet]! The casualties are too injured to move, and I need to go back inside to tend to them. Please help us!