I think this is a reasonable guess. Luckily for Lupin, Harry is pretty easily distracted (he starts to say something about the Dementors, then Snape comes in, and by the time he's gone, Harry has completely forgotten what he was talking about). If it had been Hermione, she would have noticed Snape's odd insistence on taking the potion, and probably worked out that Lupin was a werewolf much sooner than she did.