Liberty
LostCousins Megastar
Having a fact to represent these cases is probably a good idea. You can then use the Facts report to list them in one place and continue to research them from time to time. I'm assuming you can only have solid colours, but what about a reddy green or a greeny red?
There's discussion elsewhere about colour blindness - maybe not a red/green colour! (isn't reddy green a sort of murky brown?)
I like Alexander's suggestion of the tag 'Missing'. I am quite prepared to look every so often for the family of 4 small children who vanished from the 1881 census, leaving their parents in peace, (reappearing in 1891) but would rather that I had something to alert me before I started that it wasn't going to be straightforward.