The only time a Case File will disappear is if the user intentionally deletes them. If you think you or someone else may have deleted a Case File you should be able to find it in the Recently Deleted location (where it is stored for 30 days before it is permanently deleted). Open the app, and in the Locations sidebar tap on the Recently Deleted icon. If you see a Case File there that you want to recover, tap on the Select button, tap on the Case File, then tap on Recover.
If your Case File is not in the app-specific Recently Deleted location it is probably that you just cannot locate where it was saved on your iPad.
To locate a Case File open the app, and in the Locations sidebar tap on the Recents location at the top of the sidebar which shows Case Files that you have recently opened. If you see the Case File you're looking for, long press on the name and choose Get Info from the resulting popup. This will display information about the Case File, and at the bottom beside Where it will show the location where the Case File is stored. Tapping on the location will bring you to that location.
Another option, if you know the name of the Case File you’re trying to locate, is to search for it by typing the name in the Search field in the top right corner of the Case Selection screen. If you find the Case File you re looking for, long press on the name and choose Get Info from the resulting popup. This will display information about the Case File, and at the bottom beside Where it will show the location where the Case File is stored. Tapping on the location will bring you to that location.
Once you find your Case Files you should move them to the On My iPad location into a folder named with the app, for example On My iPad > TrialPad for your TrialPad Case Files.