Food making dog behave badly?

About four months ago I switched food for my dog to a low calorie low fat one since he's got a sensitive stomach (vet recommended this food for him). I opted for the low calorie option as he isn't as active as he used to be due to me having health issues. He still gets the exercise and mental stimulation he needs though. His stomach has been better than ever but he's been rummaging through the trash, even when there's food left in his bowl. He has never cared for food, he isn't a food driven dog. The trash is stored in a cabinet with child locks on, but he obviously has managed to figure that out. Do you think it could be because of the food he's been getting somehow? Nothing else has changed in our lives and he is otherwise healthy. There hasn't been anything "tasty" in the trash either. I'm just confused why he'd do this. It's making me worried as there are things in the trash that are definitely not suitable for a dog to eat. For now I'm actually storing the trash up in a cupboard so he can't reach it..