I actually see this in the shop a lot and I've always attributed it to car washes or some other contaminant. The pads are hot when the vehicle is stopped and you get those transfer marks as the rotor is cooled. That coupled with whatever the car wash is spraying on your car leaves those nice looking deposits. Brake pad deposit from stomping on the brakes (and they would have to be pretty hot) leaves more of a smear pattern and not the straight vertical lines as pictured. I get on my brakes very hard a couple of times a week to the point that the abs takes over I don't have marks like that. Anything is possible though.
It sucks about your wheel issue and you may never know what happened but if I had to guess I would say that it happened after you took the vehicle back from the dealer. Most people are pretty in tune with how their vehicle feels while driving and while we may not notice small things we do notice when all of a sudden things aren't right and that's why I think it happened after the dealer. What I believe happened is that your tires were rotated and then you started to get a belt separation. They put the assembly on a balancer and saw a wobble and said it was the rim. I've seen this exact same scenario play out before, I've even had it happen to me and one of my techs swore up and down it was a bent rim when in fact it was a belt seperation.