The tarmac scene is important for so many reasons and I believe that this under-appreciated moment is perhaps the most telling of all.
Keep in mind that this GIF is taken from the moment after Sherlock asks for his final conversation with John to be private.
Follow Mycroft’s eyes. He is visibly shocked, dropping the ambivalent mask he always wears, for a fraction of a second, and he looks down at Sherlock’s lips. And not only that, but he visibly recoils just a fraction of an inch. People tend to do that if they are not only shocked, but fearful. If there is anyone in the world capable of observing what Sherlock feels and using those observations to extrapolate what he might do at any given time, it’s Mycroft.
In this moment, Mycroft knows that Sherlock is prepared to do something incredibly stupid. Shock, fear, and looking down at Sherlock’s lips….. This tells me that Mycroft likely thought Sherlock was going to confess his love to John, maybe even be so bold as to kiss him.
A great deal of us thought that Sherlock was going to say, “I love you.” We can all rest easy now knowing that Mycroft did, too.