Leaving a large group of people with your significant other, saying you are going to "warm up the car" but are really engaging in some sort of sexual activity with them. Much less suspicious when said in the winter, but is extremely fishy in the summer.
Boy whispers to girlfriend: "Hey baby do you wanna go warm up the car?"
Girlfriend whispers back: "I thought you'd never ask."
"Hey we'll be back in a few minutes, it's cold outside so we're gonna warm up the car."
A couple leaves to 'warm up the car', and one person says to the rest of the group: "Wait, why are they warming up the car? It's the middle of summer!"
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.