Key panic is the feeling of horror one experiences when they aren't sure if their keys are on their person. It can last for an instant during a pat-down of the pockets or for a long time while one searches the house or retraces his steps through the neighborhood.
The feeling of panic may linger for a while after the keys are found, and may even induce post key panic stress disorder, where one frequently reassures oneself that they have their keys and checks for them often.
John suffered from a bout of key panic when he realized his keys weren't in his pocket. The panic ended when he found them on his desk.