That is a weird spot to get rust - I do know that the G has a water resevoir there that catches water when you open the trunk (if I'm thinking of the right spot), if the paint somehow got damaged it wouldn't take long at all for a rust bubble to form. Of course it could be a defect but my guess is just bad luck. Have them replace the deck-lid.

 

As you said... there definitely is a water-drain at the lip of the trunk lid.

It hadn't crossed my mind until you mentioned it, but I regularly clean out those drainage passages. I usually find an accumulation of debris just above my taillights where the water is supposed to drain down and around the taillights. There is a good chance that your drainage passages are clogged to the point where water was just pooling up in there and exposing the trunk metal to prolonged periods of water sumbersion.

I'd check that out and definitely clean out the drainage passages before bringing it in, just in case the dealer tries to point fingers and toss blame on you. I'm not even sure that they could blame you for that since it's not really mentioned anywhere in the required maintenance(that I can think of) to clean out those drainage areas. I'd veer on the side of cuation with this one though and just clean them out first.