There is no requirement for a carry-on check when someone offloads themselves. This was likely done to calm people down once it had been raised. To be frank, unless a crew member had eyes on the offloaded pax at all times and knew that the pax had never accessed any location other than in F, if it was necessary to identify hand luggage for F, it should have been done for the entire aircraft. But, it wasn't necessary.

Both checked and carry-on luggage are screened and, since 2003, bag matches have not been done on checked luggage either.

When the pax off-loaded, there were a series of steps involving both the Captain, the FA and the GA. Thus, the Captain was most certainly aware.