non existant page showing in google analytics

I remember having something similar happen in the GA account of a former client a couple of years ago. As I recall when investigating that, it turned out to be a form of spam, to get a link showing up in your analytic reports to the spammer’s website.

There’s another possible cause as well, which I found in this post in Stack Exchange you should also take a look at: https://webmasters.stackexchange.com/questions/79392/why-analytics-showing-non-existing-pages-in-active-pages

I recommend you follow the advice in the answers to that thread and let us know if that fixes your problem.