Maybe it has already been discussed before, but as I didn't find it elsewhere I just open a new thread now.

In the laboratory, when Molly says to Sherlock "You always look sad when you think he (John) doesn't see you"  - how did she come to that conclusion? Why does she say it?
Is that true - do you agree with this? Because I actually didn't get the impression of that...
And I also can't interpret Sherlocks view after she says that. What does he think about her statement?

Do you have any ideas?
I suppose I somehow didn't understand this scene...?

And maybe this refers to the scene when John sees Sherlock falling down (because Sherlock wants John to watch at him, doesn't he?) -
or am I just over-interpreting it?
*confused*