It's not so odd... a new carbon frame I acquired recently had the plastic guide fitted on the BB, but it is far too short to prevent the cable so it doesn't rub on the rearside of the BB wall as it changes direction upward to the derailleur. I fitted a short piece of derailleur cable outer to protect the finish and stop the cable sawing through the BB!
This frame wouldn't have been fitted originally with a tubular guide clamped on to the seat tube immediately above the BB, by chance? Seems to me that the guide has been removed and the previous owner had no other way to route the cable. Creative engineering, in my book.