Purely speculating here, but I think that the Laundries found an empty gun case in the Mustang when they picked up the car, and between what he said to them before he left, his failure to return, the circumstances overall, and the missing gun, they put it together and concluded that he had probably gone to the reserve for the purpose of killing himself. I am guessing that there may even have been a note, which would have been what gave law enforcement such confidence that the reserve was the place to look. MOO.