Not corrupt extent. Unallocated but unreadable page in an allocated extent (otherwise CheckDB would fall over it), and it's a guess.
Not easy at all to confirm, shrink may work, more likely to fall over the damaged page. Rebuilding all indexes may fix, may fall over. Moving indexes into another filegroup may work too.
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