Rare icon not showing on map

Twice today, the icon for a rare monster has not displayed on the minimap or map overlay (once in the iron citadel, once in the crypt map). In both cases, I had the entire map revealed and had killed all other bosses/rares.

I was able to find the rare in the crypt map, which was a monster burrowed underground (I forget the name, it shows up in Act 2 Valley of the Titans). It only engaged me after I walked over it.