I just finished the third book and... /what/. I mean... /what/. For two books the only thing he wanted was a) his memories, and b) Theresa, and now... like... no? Even her death wasn't really touching because she was just so hated on and it felt like... meh. Everything just felt so unsatisfactory?! Will the 'prequel' thingie make things worse or better? .-. Not sure if I really want to read it anymore...
Oh, and I just remembered... Didn't they mention a picture of a woman in the sleeping house of the gladers in book one? Something about the living room being so bland, with nothing in there but a table and a picture of a stern looking woman? Did they ever say who that was supposed to be?
Yeah, I think there's quite a lot of people who weren't completely satisfied the end of TDC. If by the prequel you mean The Kill Order, it isn't really connected to the rest of the series in a way that reading it would make you feel any different about the ending of the TDC, so it wouldn't exactly be a 'better or worse' kind of situation. If you meant The Fever Code, then I'd like to think that when it comes out it will flesh out characters more and maybe do some things that could make you see the ending in a slightly different light, but I still think it wouldn't change your opinion too much. It'll still be worth the read though.
Also no, to my knowledge it has never been mentioned. It's definitely not mentioned in the books, and I've never heard anything about it from Dashner, however I'm not 100% sure. (For anyone wondering, the quote is: 'The only decorations in sight were a dusty vase on a three-legged table and a black-and-white picture of an ancient woman dressed in an old-fashioned white dress.')