They will be in a collection - this happens in LrMobile.
So, in LrDesktop, right click the photo and you should see the collection and be able to go directly to it. Remove the photo from the collection, but leave the collection set to being synced.
Thank you for your answer
No, when I right click on the picture the message is : Not in any collection, although the "sync" icon is on.
I guess, this is because I did the following in the "Lightroom tag" of the Prefences panel.
So, my pictures are stored in the corresponding folder (defined by : "Specifiy location for Lightroom mobile images" as above): with the sync tag "on".
This is a regular folder with subfolders formatted by capture date as specified in the preference panel.
See below (it is not a collection!).
They are in no collection. The sync icon is "on".
When I move these pictures into another folder, the sync icon should disappear and the picture taken out of sync. But they are still synced to my mobile devices.
I guess this is a bug.
That folder is only about the physical location of the file, and does not determine whether it is synced. So when you move it, it is still synced.
Is it in All Synced Photographs? That's in the Catalog panel. It is possible that a photo can be synced, but not in a collection. Annoyingly, that means you can't unsync it without a bit of lateral thought. You add the photo to a collection that is synced, and then remove it from the collection. See if this works.
To prevent this happening, on your phone you should always import into a collection. Maybe just set one up called Android photos?
PS I don't have an Android device but am pretty sure this aspect is identical on Android