5301991f_and_The_Art_of_Breaking_Things__detailed_explanation
[signal] Tight-lipped smile [part of body] mouth [possible meaning(s)] secrecy or withheld feelings [detailed explanation] 
Stretched across face in a straight line, teeth concealed. The person smiling has a secret they are not going to share, possibly due to dislike or distrust. Can also be a rejection signal.
“I pretty much try to stay in a constant state of confusion just because of the expression it leaves on my face.” – Johnny Depp | Zen and The Art of Breaking Things
detailed explanation