Hulababy - big {{hugs{}}} for you!
As the others have said, happens to all of us at some point - and it is the most hideous feeling.
Summer 2002, coming back from France - we "lost" DS1 (then 4) on the boat. Unfortunately, on the crossing back we didn't have a cabin so were "hanging" around the boat. I took DD1 to the toilet, leaving DS1 and DD2 with DH. DH was talking to DD2, not noticing that DS1 had gone. DH thought that I took DS1 to the toilet with me.
It wasn't until I came back with DD1 that we both realised that the other hadn't got him
I stayed where we were sitting, whilst DH went off to look for him. As we were sitting by "Reception" I spoke to the Head Stewardess - well screamed at her really! She LAUGHED, saying oh this sort of thing happens all the time!!!" She then put a tannoy out saying "would XXXXXXX XXXXXXX please come to Deck 7" - I almost had her by the throat at this point - how the hell would a 4yo boy know where Deck 7 was - I didn't know!!!!
I was screaming at people "didn't they notice a little 4yo crying???". By now, lots of parents started looking and shouting for him. One dad was an absolute star and sat holding my hand, whilst his wife looked after my other 2 children.
All I kept thinking was that he'd fallen overboard as just before he went missing, DH had taken the older 2 up on deck.
Finally after 20 minutes, DH came back with him. He had wandered into the bar area and was playing on a car arcade game!!! Because he wasn't crying/sobbing, no-one thought anything of it!! He didn't realise he was lost - was just attracted by the games.
Needless to say, summer 2003 - I booked cabins for both journeys and practically held him hostage in them!!