In lots of older London flats, there is a guest cloakroom and then another bathroom (possibly Ensuite but not always) The idea was that the guest cloakroom was for .....guests, meaning the owners of the property could ensure their own bathroom was private and not visited by guests. Often the guest cloakroom was in the hall, so not off a kitchen or other room and actually pretty private.
Most people don't poo whilst visiting other people's houses - they do it in the morning or evening in their own homes. Of course, anyone can be caught short and some people are less, hmmmm.....regular!
Unless I know someone really well, when visiting I don't go upstairs in their home. Down stairs is for visitors. Therefore I would use the downstairs loo if there was one and only go upstairs if there wasn't one.
So, who knows why this visitor went upstairs. I probably wouldn't have given it much thought to be honest, but would actually expect that most guests would use the downstairs cloakroom. Therefore, I think the extent of criticism of Op is a bit harsh, because people seem to be suggesting a guest should feel free to go to any bathroom, whilst in fact I think that generally guests should use the downstairs loo - it's what it's for. I couldn't get worked up about one guest going upstairs if they did it once, but if they always did it, and I preferred my own bathroom to be kept private, I might just light heartedly mention that there is a guest cloakroom.
I would certainly not expect someone to be going into an Ensuite.