You have missing Pictures in Lightroom catalog
Please re-link these images first.Refer https://helpx.adobe.com/lightroom/help/locate-missing-photos.html
Then Sync them in Lightroom .See Adobe Photoshop Lightroom Help | Sync Lightroom desktop and the Lightroom mobile app
I'm missing the photos on my computer and was also wondering if I could get them back from the sync photos.
Lightroom do not sync your Original image.It only syncs a Preview of your image that can be edited on your Mobile devices.So,you cannot find missing Pictures from Synced Photos.