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!"