Can a mercenary draw agro?

Mercenaries seem to follow the same rules as our pets when it comes to agravating hostile creatures. In other words they won't get "agro" unless you get agro or are in combat.

So you don't need to worry about casting invisibility on your mercenary (in fact you can't). You don't need to worry about them getting lost and coming back with a group of hostiles following either (unless they were already chasing them before they ran off).

One way they differ from pets is a melee mercenary can actually hold agro away from you and keep your enemy from hitting you if you are in melee range. Pets cannot do this (an enemy will always hit a player in melee range instead of a pet).