Don't get hung up on the cover - it has no relevance to the engine underneath it. The A6 in your picture likely has an AMX engine under the A6 cover, just as the A8 will have an AMX engine under the A8 cover. The Passat with the AMX engine will have a different cover again, as will the Skoda Superb
The 30V V6 is an extremely stout engine so its unlikely it has been swapped, but if you are concerned then find the engine number of what is actually in the car and we can compare it to what it came with from the factory.