As I am sure you know the general rule for litter trays is one per cat plus one until its worked out which trays and how many they use.
I have been told tonight by one of the birmans that the tray was used and hes gone on a nice new bathroom rug grrrrrrrrrrrrrrrrr
I think the vet should check out there is no problem with Twiggy and then if not Feliway may help if its territorial marking.
If you see her making to pose for action , pick up quickly and put her in a tray
Lots of luck with her cos you are right that you need to stop it fast while she is still young.