Maybe a van door ( front side or rear) opened in haste ? U've got to watch these Delivery Vankers, sometimes
Alternatively a 4X4 or up & over garage door just catching the duck tail peek on it's travel.
Re: cost to fix ? I'd say cheap fix would be to get a Silver Grey touch up kit from BMW for about £12 squid and a sturdy hand on a sunny day
... expensive fix ? U'll need to probably have the uppers part of the boot lid (the Duck tail part) fully re-sprayed and colour matched and that could be £100's
