I think is quite possible that she did not. This scene could be interpreted as poor, besotted Anne, who can see nothing but Frederick, and so thinks everyone else should see nothing but him as well. After all, the fact that "...eight or nine years should have passed over him, and in foreign climes and in active service too, without robbing him of one personal grace!"

I love that line. To Anne, he hasn't changed at all in all that time - he is just as handsome, just as fine a figure of a man as ever - who could not notice him?! ;-)

I think the closing sentence may be most telling about this: "Anne sighed, and blushed, and smiled, in pity and disdain, either at her friend or herself."

It sounds like an acknowledgement to me that Anne realizes she is being foolish in expecting everyone to be as aware of his movements as she is, and to see him through her eyes.

Now, Lady Russell could very well have seen him, but if you think about it, she hasn't seen him in almost nine years, she doesn't know he is in Bath, she's not expecting to see him - given all that, I think that her eyes may well have passed over him, but who he was didn't register on her. I think if she did recognize him, she would have been so startled, that she couldn't have hid it from Anne.