is it possible to find alerts that have been made that have not been shared globally by the alert's creator, making these alerts invisible to even admin level users?
Why would some alerts not be visible to an admin in Splunk?
Perhaps you are referring to user level or private alerts, all will be visible via the top menu of Settings -> Searches, reports and alerts, even private alerts.
The private alerts will not be visible by the alerts tab inside an app, you will need to use the Settings -> Searches, reports and alerts .
It's possible the alerts creator did not have write access to the application context and therefore they could not share the alert, or perhaps they had a reason not to share?