Well, you might have just missed it, it's insinuated, not outright stated.
Yeah Tifa was definetly acting odd when you briefly jump out of the flashback. Lots of "..."
Originally I thought it was just because she didn't really want to contribute to the conversation, as given what happened to her father it's a pretty touchy subject.
But yeah, it's because she knows about Zack, I believe. She's pretty much like "huh, don't remember seeing you (besides when you saved me inside the reactor) at this incredibly traumatic event".
Watching Tifa be suspicious AF is my favorite part of playing through FFVII with my mate, I never really spoiled him on stuff (he somehow was even shocked that Aerith died, although I KNOW that's something that he should know) but I have pointed stuff out, and as a result he's been extremely suspicious of Tifa throughout the playthrough, which is fun. We're just post lifestream now though so he now knows the truth.
"One person seems to be missing"
Holy fuck
Edit: Wait, isn't Cloud actually the security guard who stays with Tifa outside of the reactor? Then was the dude who went missing just some random guy?
It'd explain why he didn't verbally respond to her.
Another favorite, spotting Cloud in flashbacks, looking at Tifa's window, talking about being nervous.
"You really, really are... you... right?"
The Tifa dialogue at Cosmo Candle is pretty great.
This really is the joy of a second playthrough IMO, everything about Tifa and Cloud becomes so much more interesting, same with Aerith and Cloud/Zack after playing Crisis Core. (Same goes for remake).
Didn't even think about that.
I've had Aerith and Nanaki in my party so I'm probably gonna get the Aerith date again.
Dude, that is such a waste, that's basically the main reason to do a second playthrough, to really see the plot as it is instead of the narrative, hope you're at least as nice to her as possible.
Ps, you mentioned something during your first Kalm flashback playthrough about watching Cloud do squats, to which I responded with "yeah, watching Cloud do Squats is always fun". Now you know why I suppose, Squats are a quirk of Zack.
This amused me when watching a playthrough once. Person saw Cloud doing that and went "He's doing Zack squats!" since they knew Zack from some other piece of media XD