Do you have torn page or checksum detection turned on the database? If so check msdb.dbo.suspect_pages to see if there have been any IO issues.

How often do you checkdb? When...

Maybe your log drive filled. Did you check the ERRORLOG?

Yes.

Are you recording login success/login failure in the engine? If so you can look in the ERRORLOG to see what did it. If you don't do this you should at...

Can you quote the part of the PCI-DSS that says this?

Not easily.

You'd need to get a developer to write an SQLCLR function which can implement the SMO assemblies (which have a T-SQL parser class, apparently, though I never got it...

Bump, this URL still doesn't exist.