[Tercioo]

At the moment if the client doesn't see the guardian being summoned (like mage's mirror image for instance), there is no way to know who is its owner.

In the other hand, if we hover over the guardian, the tooltip immediately tells who owns it, so the client holds this information and could be easily queried by UnitOwner (GuardianOrPetGUID). Its a simple thing, but, solves tons of issues.