I believe what is meant to happen is that everyone who is due to play has been entered into the computer by the match and handicap secretary, then when the people don't enter their cards the computer picks those missing players up and an NR can be assigned against them.
Trouble is most clubs don't enter the names into the computer beforehand, only afterwards when the players do it themselves.
The missing players should be advised by the committee and told they must enter all scores because despite NR'ing the holes they did score on will be used to calculate an accurate CSS. A card should always be signed and returned whatever the circumstances.