Yes, when there is a ghost update, unfortunately, there is no change in the application history section and we are left to wonder what it would be. Based on my limited knowledge, at your stage, it is most likely when a part of the BG (criminality or security) is marked as completed and the other one is still in progress. So you see a GU but since the BG didn't fully complete yet, you don't see it updated in the history section. Alternatively, it can be that the officer is working on scheduling your test so you might get a test invite update in the tracker in a few days/weeks.