No, there were four people who witnessed the killing. You're forgetting Hank. Presumably, Hank filled out a bunch of paperwork after killing someone. He would have had to inform his superiors at the DEA, and he probably would informed the local police as well.

Gus Fring was a respected Albuquerque businessman. He had extensive ties to law enforcement. He employs Mike, a former cop. He gives to fundraisers and has access to the DEA: youtube.com/watch?v=mjZBahh8ie0 (h/t to IMil in the comments for the link). He's clearly tossing a lot of money around, so it's not much of a mystery that he has access to DEA information.

No, there were four people who witnessed the killing. You're forgetting Hank. Presumably, Hank filled out a bunch of paperwork after killing someone. He would have had to inform his superiors at the DEA, and he probably would informed the local police as well.

Gus Fring was a respected Albuquerque businessman. He had extensive ties to law enforcement. He employs Mike, a former cop. He gives to fundraisers and has access to the DEA 

(h/t to IMil in the comments for the link). He's clearly tossing a lot of money around, so it's not much of a mystery that he has access to DEA information.