Could you tell us why it is, please?
Sure. From personal experience people often try and make the last night of a hol a bit different/special. In this case T9 may well have decided to include C8 on the last night, instead of leaving them 'home alone', and given GM's 'booming Glaswegian accent' and propensity for sharing info with others he may well have alerted any would be abductor it had to be 'tonight or never'.
Conversely it seems a lot of MW guests arrived on Sat 28th Apr and departed on 5th May. Any would be abductor may well have had other commitments on 4th May meaning the 3rd May had to be 'tonight or never'.
I think its important to consider the following:
Sat 28th Apr - Arrival - Millenium for dinner T9 and C8
Sun 29th Apr - Tapas T9 - C8 home alone -
Mon 30th Apr - Tapas T9 - C8 home alone -
Tue 1st May - Tapas T9 - C8 home alone - Mrs Fenn heard MM crying 10.30pm - 11.45pm
Wed 2nd May - Tapas T9 - C8 home alone - MM 'Why didn't you come when Shaun and I were crying?'
Thu 3rd May - Tapas T9 - C8 home alone - Disappeared
Fri 4th May -
Sat 5th May - Due to depart
Does the above hold any clues? Potentially it shows a would be abductor had spotted a pattern and opportunities by 1st May and started entering the apartment disturbing the children.
The other rather chilling thought is that Mrs Fenn states MM was calling Daddy. Was the would be abductor standing in the room and MM thought it was GM? If MM was crying for over an hour calling Daddy why did she not then switch to calling Mummy in the absence of GM not appearing?