I'll have to go watch it again to confirm.. But remember; C3P0 had his memory wiped at the end of the prequel trilogy (so he would have no background knowledge of her) and, although we see R2D2 with Leia, I don't know that C3P0 was formally introduced to her.
His comment about her not escaping indicated that he knew she was on-board, but if he had not 'met' her, he may have guessed that the image was her, but having no formal knowledge, chose not to state so.
I just watched it; and where R2 is getting the message from Leia, C3P0 is wandering around asking "R2D2, where are you?" before spotting him down the hall, with a somewhat cloaked Leia. He doesn't ask anything about what the Princess wants or anything, so I would surmise he didn't know who R2 was interacting with. He also has no clue about what 'Secret Mission' R2 is on (before boarding the pod, and later, on Tatooine, he asks what mission R2 is talking about)
When Luke asks him later about their background you get this quote:
He says he's the property of Obi-Wan Kenobi, a resident of these parts. And it's a private message for him. Quite frankly, sir I don't know what he's talking about. Our last master was Captain Antilles, but with what we've been through, this little R2 unit has become a bit eccentric.
Give that Threepio thinks of himself as belonging to Antilles, it's not unreasonable to assume he may not have met the princess directly.