Lots of questions! I'll try and answer them all as best I can. I have emailed both ancestry and mundia today in an attempt to get it sorted.
The person who contacted me is a mundia member and was exploring one of my Ancestry trees on Mundia. They messaged me via the mundia/ancestry messages.
I have been an ancestry member for some years now and have a few different trees, one of which is private. I assumed from the ancestry reply, that everything was now unsearchable on Mundia.