Thread: Innapropriate Pee`ing

Any advice please.
I have 3 adult Maine Coons, 2 girls and a boy, together since January. Always very good with using litter trays, indoor cats with access to an outside run. Over the last couple of weeks one of the girls has started peeing everywhere. She has peed on nearly every chair in every room, in my sons bed, in the other cats baskets, even in her own. She is still using the litter trays for "routine" toilet use, but if she can get to a chair when you are not looking she will pee on it, very furtively. If you spot her it seems she knows she is doing wrong and runs away.
HELP!!