JCandeias, thanks for advice - Backspace somehow did not work this way - although I never changed the hotkey settings...
True. I wrote that without the program running, off the top of my head, sorry. Turns out, backspace selects not the object you were using as reference when you lost focus, but the one before that (and the one before THAT if you hit it again, and so forth). I guess it goes through your travel log, backwards.
Anyway, what I do when that happens to me is either to use the planet browser when I'm on a planet or moon, or to look down until I'm able to reselect the object if I'm on something else.