CutiePie--The sororities definitely know who they're snapping. They're given a list of all the girls who didn't get bids. At most schools a sorority can also snap a girl who dropped out before Pref. (Doesn't matter if she choosed to drop or if she was cut from every house.)
At my school the chapters recieve their pledge class list first thing in the morning. Then from 11am-1pm all the chapters who didn't meet quota are allowed to call girls and offer bids. From 1pm-2pm chapters that met quota but not total are allowed to call. Before 11am the girls Rho Chis are supposed to call them and let them know what happened. Unfortunately the year that I called a girl to give her a bid, her Rho Chi hadn't reached her yer!
That was a little akward.
The snapped pledges then come to bid day just like everybody else does and the rest of the pledge class is pretty much clueless unless they choose to talk about it.