I've set up a mailing based on a search. A Hidden Smart Group was created, just like it should. If I schedule that mailing to go out a week from now, will the hidden smart group update and include more recipients based on the search criteria before it goes out or will it only contain those contacts that were a part of the first search? Thanks.
As of today the recipients are fixed once you schedule the mailing.
You can experiment with https://github.com/3sd/civicrm-recalculate-recipients, if you want the recipients to be determined once the mailing is sent.
As per https://lists.civicrm.org/lists/arc/civicrm-dev/2017-12/msg00012.html there are reasons to prefer both variants.