Sorry but I can't believe it will be Lilly, Duck or Kenny. It's just too obvious. It can't be Clem because she was so against stealing from that car. If someone is stealing from the group it has to hit home for Lee story-wise.
I think it has to be Doug/Carley that has been taking the food. In episode two, both Carley and Doug turned down when you offer them rations which I found remarkable. I really like Carley and it will bug me to no end if she's the one that has been stealing food but regardless it will be in the spirit of the TWD. I'm curious to see how such a situation plays out and how I will handle it... Probably not well.