Merry Christmas.
Maybe you're getting a new bike for Christmas? :-)
Are you visible from the road going in & out of the shed, or was it just opportunistic?
Adam Hart-Davis had his bikes pinched recently from an outside shed I think, even though he had some special locks bolted to his shed.
Very sad at Christmas time. I've even read cases where thieves had stolen children's toys.
Good luck in finding a replacement 20.