John is an observant man. He may not be quite as observant as Sherlock, but, in specific areas, he's got some game.
He knows, for example, that Sherlock can go for about 76 hours between food and faint. He's also learned that if someone was to drop a biscuit, or apple segment, or so forth into Sherlock's hand, Sherlock will generally put it straight into his mouth, chew and swallow, just to get the use of that hand back.
On one quiet evening, just for the fun of it, John had reviewed his CV and included; 'food dispenser' alongside; 'sounding board', 'moral advisor' and 'general dogsbody'. He didn't yet know that Sherlock had found the file and added 'companion', 'friend' and 'sex monkey' as well. John's observation skills are narrowly channelled.
The food didn't always prevent the faint though, and John had turned his observation up a notch. It was his right as partner and his job as a doctor to check that there was nothing more sinister than low blood sugar causing these moments of unconsciousness. When he had spotted the cause, he'd almost laughed with relief.
This is why, at critical moments of intense brainwork, when Sherlock has focussed all his energy away from the 'transport', John will walk gently past his friend, and softly mutter the word, 'breathe'.