Probably related to the alarm/theft prevention system.  Have you checked the
owner's manual to see what it says?

I don't ever recall a red "key" though the supplemental restraints light
might look similar to a key to someone with my kind of eyesight... or
perhaps the exclamation point of the brake warning light...

If you look in the owners manual, it might help.... If you don't have one, I
think you should be able to downoad one at myford.com