This question already has an answer here:
In Attack on Titan (anime), up till now we are given information that the "dumb" titans that wonder outside the walls are probably humans. Given that both "smart" (Eren/Annie/Yumir/etc) and "dumb" titans share the same weakness this has always been a possibility.
However, its not clear why the Survey Corps never finds the bodies of the humans inside the "dumb" titans? They should at least find the bones of the human. Is this a logical flaw of the series?
BTW: please, no spoilers from the manga, only info available in the anime up to season 2 episode 12.