What is this mysterious Google Maps popup?
Turns out the perpetrator is - for whatever reason f.lux. 
The window is truly a zombie though and even stays unmovable and open if one of the buttons is clicked. Will inform them. 



EDIT 17.02.18:
Here's their answer:

Thanks for your note - we have fixed this but haven't updated everyone yet.

Make sure you have 39.987 (download again) at

https://justgetflux.com/

The location popup says "f.lux" now and explains what it's doing....