I agree with your thought process. I don't recall reading anywhere that she brought a mug. I have a 20 yr old brother and I know at his age...most don't plan that far ahead (he would forget his head if it wasn't attached. He is a very intelligent kid, 4.0 GPA but lives for the moment and not good w/ pre-planning ). I know she made her costume but that is different since woman seem to be more into their appearance. Honestly, when I was Chelsea's age...I would not have brought any alcohol. In my opinion, it is much easier for a woman to get alcohol at an event like this. I am not saying this is smart but when your younger, you aren't always thinking about the consequences of your actions. I feel someone could have used this as a way to get to know Chelsea and as a way to take advantage of her in some way. Additionally, maybe someone put something in her drink. From what I understand there was a large range of drugs being used at this event.
Anyways, I am sending my husband on a mission this evening. He is going to Pilot to look at all the travel mugs. He feels it looks very similar to ones he has seen there before (but who knows..I don't know that he has really ever looked at Pilot travel mugs before). He owns a company (w/ lots of equipment) and this is the only gas station he will use. I will let you know what he finds out. I asked him to take a picture of anything similar.