A Dangerous Reaction
Travel to the uranium mine
Investigate the uranium mine
Investigate Bobby Joe's campsite
Investigate Bobby Joe's tent
Replace the sign
All of the locations are marked on your Huntermate, though I have no idea what was up with the sign! I don't know if I picked it up without seeing it, or whether it just wasn't actually there. It certainly wasn't there when I "replaced" it!
Completing this mission will unlock the trophy/achievement Better put this back up.