"I thought the priest wasn't supposed to know who was on the other side of the grille. So Technically he doesn't know who is confessing the sin."

The standard confessional "booth" is indeed set up to provide anonymity, or at least the illusion of same. For a regular churchgoer making confession at his home parish, of course, it is fairly likely that the priest will recognize the person's voice and perhaps even his/her predelictions (sins).

When I was in Catholic elementary school at St, Mary's, Plainfield, NJ, I remember once going to confession, finishing, saying the Act of Contrition while the priest recited the Latin words of absolution, and then having Fr. Moran ask me, "So Tommy: are you the last one or is there anyone still waiting on line?"

More recently, there has been general discussion that people are encouraged to meet with priests "face-to-face" for confession, making a better setting for personal counseling (I suppose). The standard "blind" confessional booths are still available at regualrly-scheduled hours, nonetheless. Longtime "sinners" who become penitents after years away from The Church always have the option of going to a church where no one knows them and confessing in the darkness and from behind a screen.