Heres another vocabulary post!
Cumption (cūmp'shūn) n. 1. the ability to accurately sense when a person or a couple are postcoitus 2. knowing someone just fucked
Ex: "My cumption tells me that the secretary just had anal sex with the boss."
So, now you have a word to articulate the feeling you are feeling when you see a couple display weird behavior (in a dark and secluded area). Bug Out!