Exhaust manifold bolt breaking?

[Olfart]

at about 48,000 my 2004 went in to replace the power steering pump and idler and pitman arms. They noticed that there were 2 broken ex manifold bolts. I had never heard any hissing or other sounds indicating a manifold leak and I'm not sure which bolts they were. But in order to extract one of them they had to pull the head to get a straight shot at it. Luckily I still was under extended warranty, but the truck was in the shop for a week.