Matzah sold as Shmurah means that Jews were present from the time of harvesting, making sure that the grain stayed dry and didn't become chametz, and intending for the purpose of the mitzva of matzah.
"Regular" machine matza, if I understand correctly, is only "guarded" starting at the time of kneading. But I assume they'd only use flour they have 99% (99.999%?) certainty has not come into contact with water. How/where do they find such flour?
i called the OU today (3/10/16) to ask this very same question, and the answer i got was not amazing. They said that their normal certification for normal kosher for passover for flour is only "guarded" from kneading as you've said. i asked what about the opinion that you can't use normal flour from the market since water might have come into contact with it at some point. He immediately put me on hold for several minutes and then returned to say the following.
That the wheat is cut as normal (as in, the wheat could be rained on after it's cut), and then it is taken to the mill. And that the mill has mashgachim there, and so it is always supervised until it reaches the matzah factory, and once it reaches the matzah factory it is guarded from kneading for the purpose of the mizvah of matzah. When i asked if he could tell me for sure the entire mill was a dry process he said he was not there so he could not say for sure. i asked him for the information of the mill or the mashgachim that were there so i could verify, and he said that information was confidential and could not be shared. And we went back and forth a few different ways and it always boiled down to confidentiality. When i asked him if the mill was a dry process he said yes, but did not indicate that this process was different from how any other flour is treated or processed.
I think they would start by researching/confirming where the flour was grown, and if it rained there after a certain point in the growing process (after which the wheat can become chametz). Then they make sure that the processing of the flour did not include water (during the cleaning/grinding of the grain). I assume they'd also make sure the grain/flour was no stored outside or in a leaky silo.
The difference from shemura would be that shemura is actively watched/inspected at all points in the process, whereas the regular matza is a post facto investigation
Just before pessah I saw this video which was a news story on a few Satmer Hassidim who go up to up-state NY and supervise the harvest. I Also saw this one describing how the baker keeps the wheat in silos outside his bakery.
I was looking to show my children how hand shemura was made.
