Mark sounds lilke Willow is trying to cover the unfamiliar smells in the house.
My mum has gone through similar problems with one her's recently. As you know we have been on holiday and while we were away Mum's friends have been looking after the girls .... normally this isn't a problem as Thelma (mum's friend) has popped in twice a day and emptied their trays and everything was OK ... she has been doing this for the past 6 or so years. However, this time Thelma and her husband were having a new bathroom fitted while we were on holiday so Mum said they could have their baths and showers at her house. For some reason Lucy (my mum's cat) wasn't happy about this and took to peeing on the bed, on the sofa and various other places. Thelma cleaned up for her but Mum still had to throw out two duvets and a blanket on the sofa. Lucy was taken to the vets when Mum got back home and given a clean bill of health ..... since Mum and Dad have been back home there has been no weeing and the girls are as happy as pigs in clover again. We can only assume they didn't like strange smells in their house!
May this is why you are having problems with Willow .... she doesn't like you having guests / or cleaners in HER house!