In Home Alone, Mrs. McCallister suddenly realizes Kevin is gone once she boards the plane. The moment is abrupt, leaving audiences to wonder what triggered the busy mom to finally remember her youngest child who was left behind. She engages in a conversation with her husband, but, after a moment of silence, she lurches forward in her seat, the truth dawning on her as she yelps Kevin's name.
Kate McCallister (Catherine O'Hara) is certainly not a terrible mother, and seems to be the caregiver for the entire family, including her husband who can't manage to pay the pizza guy or get a voltage adapter on his own. Everything about the hectic morning as the family hustles to make it to the airport made it seem realistic that such a mistake could happen. The electricity goes out, making the alarm clocks ineffective, and when they wake up late, chaos ensues. Kevin slept in the attic, so doesn't hear the hullaballoo below, and everything goes awry. Leaving a teenager in charge of the head count may not have been the best idea, but it makes sense for adults wrangling all those children and going over last-minute checklists while failing to notice that Kevin's plane ticket was thrown away with wet napkins. They race off in two separate vans, each adult thinking Kevin is in the other vehicle. The airport is crowded and there is no time to stop and check to be sure everyone is there, and the kids are in coach while the parents are in first class. Every little detail is set up to make Kate's blunder not only understandable but relatable.
Anxiety causes forgetfulness, and one important thing Kate forgets in that whirlwind of a morning is that she didn't pack Kevin's suitcase (except for perhaps his toothbrush, which he can't find), as his siblings reiterate again and again that he can't do it himself. She also forgets to wake him up, a task she would likely have to do since he was in the attic. Kevin says he doesn't want to share a bed with his little cousin Fuller who is a bedwetter, and Kate says that she will move Fuller, understanding Kevin's distress. This last-minute arrangement meant that Kevin was alone upstairs, with no soggy cousins or aunts or uncles to wake him.
There is a phenomenon called "tip of the tongue memory" wherein a person realizes something once the brain calms down. For example, a person trying to solve a specific clue on a crossword puzzle may set it down for ten hours and return to it later only to have the answer pop right into their brain immediately. The same is true for details forgotten in a moment of disquiet. Once Kate is able to sit down, she goes over a checklist with her husband ("Did I turn the coffee off? Did you lock up? Did you remember to close the garage?"). To this he says no, thinking that must be what they forgot, but Kate's brain is still disturbed. In that moment, it is likely she remembers that she did not pack Kevin's suitcase, the one thing she was absolutely going to have to do for him.
It does not seem like magic or intuition that causes Mrs. McCallister to remember her son. Her husband doesn't manage to remember, leaving Kate to her restless thoughts as she sits quietly in her airplane seat. Kate's failure to notice Kevin's absence was not only brought on by a series of human error, but by a trick of the brain, the same trick that helps her suddenly realize her son has been left Home Alone.