Thanks for the clarification. Herpes is transmitted by kissing and by sexual activity.
Yes, the IgG could certainly have missed an old infection. And yes, because HSV 1 recurs so infrequently, it is quite common to have no symptoms for years and then have an outbreak. BEcause you are both negative on the IgG test, the test could have missed it in both of you OR it missed your infection and she is negative because she is newly infected and not enough time has gone by for her to have antibody.
It does require direct contact yes, and for genital HSV 1, the most common source is oral sex.