The "(REV)" in the description indicates it's a reversing entry so it's likely this was a CM transaction that was deleted. When an entry is deleted in CM, the Transaction record is removed, but there is still Cash Detail records for it.

Use CM Inquiry > Entries > Cash Detail by Batch and drill down on that batch to see the cash detail records.

Thanks. That makes sense, but shouldn't it now show up as outstanding in the CM > Tasks > Reconcile area as outstanding?

Not if it's been deleted. If it shouldn't have been deleted, it will need to be re-entered.