G-TUMF,
They say a picture paints a thousand words;
I have no further information, but I suspect whatever work is being done, Boeing is getting the invoice. The aircraft had only been delivered new a couple of weeks before the worldwide grounding, and hasn’t flown a revenue flight since. So it’s impossible to believe the work is due to damage during flight. More likely to be mainly paint defect warranty work plus assorted other issues as a result of long term storage.