It's likely that's how Bungie are labelling higher level enemies, As like in other games when you are not high enough levelled you can't see what level some enemies are and just have a skulls instead of a level number. If you also go fare enough left you will also find some fallen with the same level ????. Also Bungie could of put them there to stop us who got into Alpha from going any further seeing what's round the next corner, as the level cap in Alpha is 8.