I believe there are two notifications which looks similar, but internally goes through different execution routes: local notification and remote push notifications.
I don't have exact answer. Couple of ideas, direction, where I would look for:
API's in private framework SpringboardServices
- Objective C SBSPushStore class
- Bunch of C API's SBSPush*
There is ApplePushService private framework. I believe it provides a connection through which remote notifications are received.
Also, there is a BulletinBoardService. I believe this one is related to showing the list of notifications. My guess, that it may store notifications.