They're walking through the International terminal at Heathrow, Sherlock stalking off, leaving John to deal with his bag. When they get to the security checkpoint, where John can go no further, Sherlock turns to say goodbye.
"I'll only be gone a week, John. Possibly less, if I can sort the case out quickly. I'll be home soon."
"I know, Sherlock. Promise me you'll be careful."
"Aren't I always?"
"No, Sherlock, I'm positive that's entirely the wrong word for what you are." John smiles wistfully, adjusting the lapels on Sherlock's coat. He clings to them for a moment before letting go and looks down at his feet. He feels Sherlock's finger under his chin, guiding him to look upwards.
"I will come home to you, John. I will make sure I don't get hurt, don't get stuck there, and I will come home." He kisses John's forehead gently and turns before either of them can say another word, heading through the gate.
It's been years since the incident at Bart's, since the fall, but somehow the pang John feels every time he sees Sherlock leave him, no matter how temporary, never gets any easier. He knows it's irrational, he knows it was necessary that one time and won't ever happen again, but part of him is still terrified of being left behind.