Newt finds a new creature to add to his collection (I mean u can go with a frog if u want but any creature will do) and has lived with it for some months when he happens to one day randomly give it a lil' kiss

and surprise! it is actually the original Graves, who everyone else had presumed dead, finally freed from Grindelwald's spell which had trapped him in that body with no way to tell anyone about it.

bonus if Graves has spent so long in his animal body that he unconsciously picks up a few of its mannerisms even now that he's human, and Newt the big weirdo finds that attractive.