The radio...

Everyone was on the same channel. A radio was lost at the jail. Tardust and Scratch could have found this radio and listened to the conversations. The trick is giving them situational awareness to know that something went down at the jail and investigate. They could have a surveillance system set up just like Michael did. In addition, the wrecked gun truck and dead behemoth are a dead give away. Then, Nick, I mean Lester, changed the encryption of most of the colony's radio. I say most because, Burt said they would stay on the old frequency... Bad move...

The cut scene with Tardust and Scratch listening to the radio could easily have them listening just before and after the re-encryption of the radios. It is possible that they continue to listen in hopes that the colony returned to that frequency. There are a lot of conditionals in this, but not out of the realm of possibility. There are not many episodes left and some how these story arcs have to cross again.

Another way is with all of the people away from the colony, they slip inside and investigate the main office to see if there are any clues to where people went and notice the voice mail button flashing.