They aren't sitting in the survey's outbox are they? However, since you pulled the sqlite database and they aren't anywhere in there, it's not promising. Did they hit submit and they disappeared, or did they get a successful submission and then the data disappeared? You might submit a support ticket if you haven't already.
You can also go to your feature service's Rest Directory and Extract Changes. If they were submitted to the feature service and then were deleted, the data should show up in there. Here's a guide on how to do that. If you have sync enabled though you won't have access to the add features operation, but you can still get the data and manually re-enter it.
https://support.esri.com/en-us/knowledge-base/faq-can-an-overwritten-feature-layer-be-recovered-in-a...
It's been a long time since I've had survey data disappear and it was usually caused by mismatched versions of the survey between what was on the device and what is published, or possibly wildly different versions of the software. Also, if your settings allow for users to delete records it is possible someone may have done that. It would be a good time to double check your settings in AGOL, Survey123 website, and Connect (if you published it from there). Make sure "use latest version of this survey" is enabled. Make sure Survey123 Connect and Survey123 are all up to date. Use the Analysis tools in Connect to update the XLS form too.
