heres things that stand out to me
the tech saying it happened approx 3 weeks ago - how does the tech know this, there is no way to tell , noise would have been heard when the tech was test driving the car
and that this happened when the engine was being washed, ask yourself, if this happened at idle, how did the rod cap blow through the oil pan, this certainly wouldnt happen at idle, think about how much stress there is to an engine at idle compared to say 5000rpm, why would rprime bother mentioning that it wasnt rev'd about 3500, was it to cover themselves ahead of time? and if the 3rd party inspector is going by the words of the people in the shop, then of course hes gonna say it was a faulty bolt cuz its easier to say that to investigate properly
your car seems fine when you dropped it off, no mention of weird engine noises when you dropped it off by you or the tech, all the sudden, a rod bolt cap stretched to the point of snapping while in their possession, and not just snapping and falling in the pan but snapping and blowing a hole in the pan
maybe they performed an italian tune up with your service
http://en.wikipedia.org/wiki/Italian_tuneup