Checking in a book will always add a "return" stat to the statistics table, even if the book was not checked out in the first place. The only exception is if a "localuse" stat is recorded instead (when RecordLocalUseOnReturn is enabled). It seems to me that a return should only be recorded if the item was actually checked out in the first place. This has been the behavior for a while, though, so we should get feedback from libraries before changing it. One thing that has been mentioned is that the "return" statistic can be useful for capturing things like an item being scanned to fill an on-shelf hold. Should we make a new statistics type for local handling like that, which some libraries might want to track but others might not want to count as a circulation stat? Use action_logs to track that sort of thing instead? See also bug 9717 for a similar discussion regarding the "localuse" stat