Enemy zerg showing up as orange dots?
I got this a while back and did some research. It seems to be the common opinion that this effect takes place briefly after a player transfers from one map to another by using a waypoint. Commonly, when waypointing to the defence of a keep, from, say, a borderland map into the EB map, this can occur. Once those dots move out of range, they are not replaced and soon, the effect fades.
I had this once when an enemy thief got the stealth trap revealed debuff in a keep of ours. Tracked him on the minimap.
Would be a cool feature for roaming rangers, if they could either mark players or lay traps that would let them be tracked for a bit. Especially if rangers ever got a camo skill.