A deceptive person will often hide her mouth or eyes when she’s being untruthful. There is a natural tendency to want to cover over a lie, so if a person’s hand goes in front of her mouth while she’s responding to a question, that’s significant. Similarly, there’s a natural inclination to shield oneself from the reaction of those who are being lied to. If a person shields her eyes while she’s responding to a question, what she might well be indicating, on a subconscious level, is that she can’t bear to see the reaction to the whopper she’s telling. This shielding may be accomplished with a hand, or the person might even close her eyes. We’re not referring to blinking here, but if a person closes her eyes while responding to a question that does not require reflection to answer, we consider that a means of hiding the eyes, and a likely deceptive indicator.
Also worth mentioning is Synchrony. When deciphering truth from lie, watching for synchrony is key. Synchrony should be present in the way a person moves their head. You might notice a person verbally saying “yes,” but shaking their head “no.” If what they say from their mouth doesn’t match with what their body says, you have a liar on your hands.
Sherlock is verbally saying “You saved my life” while he is shaking his head in the ‘no’ gesture. Therefore we can assume that Sherlock is lying.