Treants do not seem to be able to see in the dark. Hence if you're a druid not wearing anything glowing and your opponent isn't either, the Treant cannot see you to rescue. Nor I think to assist you (still trying to figure out why it didn't autoassist during a PvP fight yesterday and I think not seeing in the dark might be it).

I've noticed in the past that other types of called mobs seem to be able to see pretty much everywhere and even through invis, camo, etc.. So I'm thinking this is just an overlooked bug.

And most called mobs cannot see pretty much everywhere.