Depends on when they discovered the key was missing. If the key had disappeared for several days and then suddenly turned up again, then they should 換門鎖, just in case. If the key was recovered within hours, they probably thought that the girl had just carelessly misplaced them. Then, what you said makes sense.
@@hkhistoryw5062 in my place, it only takes half/an hour to duplicate a key or remote key. Probably the student didn’t even realise the key was missing, like the teacher might return the key by the end of the lesson
