In The Martian, an astronaut stranded on Mars finds and boots up Pathfinder to contact NASA. Thankfully, they’ve been watching him via satellite and figure out what he’s doing, so they’re ready for the signal.
But I’m curious to know what would happen if they weren’t expecting the call. Would a blinking light go off somewhere, or would no one notice a signal coming in if they weren’t looking for it?
If I recall correctly from the book, they had to go to the place where it was made and pull everybody off of whatever they were working on so they could figure out how to receive a signal from it. Maybe somebody had to get pulled out of retirement? The book was fairly solidly-researched (except for a few fine points without which the author wouldn’t have had a story to tell), so I’m guessing everything out there has its own frequency it transmits on, and it’s a very faint signal from far away and if you’re not looking for it exactly, you’re not going to catch it.
NASA’s Deep Space Network is about the only thing that would notice. And since we always have a Martian mission or two active these days, at least one antenna of the DSN is usually pointed in that direction, so it might detect the sudden activation of an old probe. The only question is the frequency. They’re monitoring specific frequencies that active missions are using. Does the old probe use any of the ones that the currently active ones do? If so they’d notice; if not then probably not.
As far as the book, pretty much all the people who’d worked on Pathfinder had been retired, so they all got temporarily reactivated.
Pathfinder’s signal and baud rate were so weak that it would be ignored as background noise unless being specifically listened for. Current rovers communicate with NASA by use of relay satellites in orbit around Mars which transmit a much more powerful signal.