Smell! Observation! Behaviour! Reaction

I dunno if this is a myth or its real?? But some guys or girls are really good at observing behaviours and percieving particular smell tht tells if somone is horny or just had sex!
If you know about this tell us!

How does a horny boy/girl smell or behave like??

How does a "just fucked" boy or girl smell or behave like??