I'm not sure what could cause this, I have passed this on to an expert on corruption, don't know when I'll get a reply.
Can you try and run the checkDB on TempDB? I don't know if it's allowed on that DB or not, but is worth trying.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability