Thankfully the baby was okay.
I can see how this could happen. The only reason it hasn't happened to me with our 5 kids is that I am extremely conscientious to the point of being obsessive. Most of the kids are grown now but in the past I have done a head count a thousand times to make sure we had them all. And, I used brightly colored clothes on the kids when we were out at a public place or event.
But, one time when my 4th was a month old baby, some friends from out of state were visiting and we all decided to go to town for ice cream or something and I happily headed out the door towards the car and the other mom exclaimed, "Isn't someone going to get the baby!!??" For a few minutes I actually forgot I had a baby. We all laugh now about that but it freaked me out for awhile!
Weird things happen.
I can hardly fathom keeping up with five children. I only have two and we broke a record last year - I quit holding their hands walking across parking lots from vehicle to establishment. They were certainly happy campers considering they were 18 and 19 at time. I honestly believe they complied for so long just to humor me. ;-)