I think the "coldies" were picking up the magnetic burst, not Desmond's (Libby's) boat's signal. It's pretty clear right?
So how did they know to look for a magnetic burst? Well, Desmond's girlfriend, who's rich and persistent, is funding the search for Desmond, so...
Obviously, Desmond's girlfriend's father has a hand in the island and the Dharma project. In fact, he rigged it so that Desmond would end up there, since he wanted to make sure Desmond never saw his daughter again. But Desmond's girlfriend found out about it and hired some dudes to investigate. We'll see lots more of Desmond's history to flush this out next season.
So how did they know to look for a magnetic burst? Well, Desmond's girlfriend, who's rich and persistent, is funding the search for Desmond, so...
Obviously, Desmond's girlfriend's father has a hand in the island and the Dharma project. In fact, he rigged it so that Desmond would end up there, since he wanted to make sure Desmond never saw his daughter again. But Desmond's girlfriend found out about it and hired some dudes to investigate. We'll see lots more of Desmond's history to flush this out next season.