I'll take a common, current example: Biosphere in Shipyard.

You cannot even see the first mob group from the start and yet their threat level is determined once a map is created.

Thus, if a L31 creates a map and a bunch of L35s join, the mob will have a blue threat level for the L35s. As a result, those L35 players are losing out on xp for that first grouping.

I think the radius for a mob's threat level should be smaller than that. At the very least, a mob should be in the line of sight before it can have its threat level determined.