In season 4 episode 6 of Breaking Bad we see a Pollos Hermanos delivery truck attacked by cartel members. They leave the truck a shot up mess with no survivors. This event would have brought a lot of suspicion on the franchise and may have given Hank grounds to investigate more aggressively. How was this truck never reported to law enforcement?
When the truck was attacked, the men killed the guards of the meth and stole it from them. Gus, being a kingpin on the meth, probably had specific times that the men needed to deliver the meth. The fact that the truck never showed up on time, made him the first to know about it, and since it was on a discreet route through the desert, it probably would have taken a few days for anyone to find it. Also, at that point Hank was the only main person investigating Fring and Los Pollos Hermanos, so even if the truck was found, it would have been reported to the police. With the meth gone, they probably would have just thought it was shot up for another reason, and it wouldn't have been reported as a drug suspicion to the DEA. Gus would have taken care of the problem before Hank would find out about it, and would have made a story to cover up the meth.
I think it's really simple - there is no way Gus Fring would ever let something like that get to the police. Do you think he would arrange a meeting with the Cartel immediately if there was any chance of the police snooping around? In his empire, everything was organised, cut and clean. A fantastic businessman, you have to agree. Another thing to consider - it took 2 years for hank to start suspecting his obviously mad brother-in-law, police incompetence is at serious play here. One more thing - the Cartel guys are professional hijackers and murderers, they did everything in style. More about the truck hijacking here: http://breakingbad.wikia.com/wiki/Gaff