Excel has an annoying habit of making the current selection invisible when it becomes inactive - including the "focused cell" rectangle. When I try to process a spreadsheet in a certain way I may put it on a secondary monitor and highlight the stuff I'm working with, but the moment I switch to another program (on the primary screen) I can no longer see where I was.
It must have made sense 10 years ago before lots of people had multiple monitors, but now it's just an annoyance. A quick google doesn't find anything at all on this.