If you have one of these spies on your ship that was placed because someone completed this artifact, does this essentially mean that you have to trade in your ship that it has been placed on ? I ask this because its cloak is soooo high... Or is it possible to detect it if you change ships and purchase a high sensor level ?

Cloak will be based on the owner of the ship who placed it. Yes they are detectable like any other spy is, its no different. If your running a small sensor ship then you may need larger sensors to see it. Again its all dependent on the guy who placed it, no different that if you picked it up off a planet you captured.