Hi Walter:
I know the red filled box should be enough to  get our attention when GPS signal is lost, but I confess that I missed  it in flight for about 15 minutes on my long AZ FL trip. (It was a long stretch  over TX, zoomed out a bit, where nothing changes much.) I only noticed  it after I realized my own-ship hadn't moved for a while. (And all the  time I thought I was holding course perfectly! <grin>)
Brainstorming:  Perhaps a flashing "No GPS Position Lock" box, although I think most of  us are learning to tune out flashing ads on webpages. Maybe you could  move the Red box around the screen, like an anti-burnout screen saver  moves? That should get our attention. A "black/yellow" chevron like border flashing around the whole display? Own ship blinks by alternating to an giant X? (Or a Mr. Yuk face? (Just joking.))