I mean, I'm not surprised she hasn't made the connection, given the brevity of Frank's description:
“Big chap,” said Frank, frowning in recollection. “And a Scot, in complete Highland rig-out, complete to sporran and the most beautiful running-stag brooch on his plaid.”
—Outlander, chapter 1
That could be any of the MacKenzie men... (FWIW, the brooch is only mentioned one other time, that I could find. No idea how common a motif it was in 18th-century jewelry.) I guess there's always a chance Claire will remember in a future book, though.