Not sure why CheckDB's not picking this up, but the following should fix it. Do this in a maintenance window if possible.
Full backup
Switch to simple recovery
Checkpoint
Switch to full recovery
Full backup
Log backup (and see if it works)
There's an article on Paul Randal's blog on, I don't have the link handy.
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