The answer I got was that for a large company, hiding things on the balance sheet is like getting a home inspection and finding out later that the house has some big problems with it. The point being is that inspections find somethings but can never find all issues.
What they meant was that if you dig deep enough you can always find out the problems, but when your going through the aquisition, you often don't have the proper access to the records you want, or the time to do the entire audit.
HP has owned autonomy for over a year and just found out this quarter. There is no way they would have had such access to autonomy's books during the due diligence phase nor is there anyway that autonomy would have let them take over a year to do their due diligence