@Fends "I am not understanding your point that a flight with a possible capacity for 300 and generally flying with around 90% full seats, would be alarmingly and obviously missing passengers because 10% of its seats were empty" 🤔
The cabin manager will have been told exactly how many passengers should have boarded based on the number of scanned boarding passes. If the flight was "full" then the number of empty seats should have been noticed. If the flight was "90% full" then it should have been clear that there were 70 empty seats instead of the expected 30 empty seats (on a 300-seat aircraft). The cabin manager is the most experienced cabin crew member and should have enough experience to notice this discrepency and in any case, if the final manifest states "270 passengers" (or whatever) then the cabin manager has to be satisfied that this is the actual number of passengers actually on board.
Modern aircraft also have automatic weighing systems - i.e. the pilot can see the actual weight of the load (passengers plus baggage). If there are 38 passengers missing, then the aircraft will be showing a discrepency of around 3,200kg compared with the expected weight (38 passengers x approx 80kg). Pilots are supposed to check this, usually as a precaution against trying to take off when overweight, but should also query if the system is showing a weight that is far under the expected load weight. Either some of the passengers or luggage is missing, or the system is possibly faulty.
Do you really want to fly on any aircraft on which the crew does not know exactly how many and who is on board?