Radio Tag

Radio Tags are seen in Red Faction: Guerrilla. They tell some backstory of the game and the games before. There are a total of 36 collectible Radio Tags. They can be easily identified with an orange-ish fog over them and a skeleton. They can be seen on the minimap with a green dot that appears every time the radar scans the area twice. Some of them are quite humorous while most are depressing. If you collect every Radio Tag you get the achievement/trophy "Lost Memories". Some of the tags are story lines from Red Factions 1 and 2. There is even a radio tag from Alias.

http://www.msxbox-world.com/features/Red-Faction-Guerrilla/index.php is a good resource for finding these.

Notable Radio Tags
On the mission "Ultor Echo' there is a radio tag where you can hear Capek talking to Parker when you are inside of his science facility.