I think in the *books* he had remembered how Harry opened the Chamber waaaaaay back when...
Which is totally silly because a) JKR makes SUCH a big deal out of how unusual it is to speak Parseltongue and that it is a talent not a learned language and b) I can't remember exactly how someone said something in *English* if I'm trying to recall the conversation the next day-- several years later in hisses? Yeah, right.
The movie version is even DUMBER. How does he know Harry wasn't having erotic dreams and hissing to his partner? Maybe he was telling the door that it was really bloody hot? SO dumb to even think that Harry has dreams about opening doors, in Parseltongue and then talks, in Parseltongue in his sleep... and that there would be *any* way for Ron to know what the hell was going on.
Pir8fancier did a couple of essays on major book 7 plot holes and that was the biggest I think. I think her usericon is still "Parseltongue is not f-ing French" (Although I do think she spells it out.
Anyway, third party Legilimancy makes a hell of a lot more sense than any of that. At least it was *always* established that Severus was a Master of mind magic, so good that he was one of only a very few who could not be read by Voldemort...