I'll just expand on that a little bit.
When we viewed the data, there was 18 gigabytes of it. That's about four DVDs' worth of data. That would have been intermixed with hundreds, or perhaps thousands, of gigabytes' worth of photographs collected at the same time. Google cars use a proprietary storage mechanism to compress all of the information they can onto as few hard discs as possible while they're driving around. All of this information is being heavily compressed and written to the hard drives within the cars, so it is possible that they just did not see that data there in among all the other data that was there.