its rare but occurs maybe once or twice a year, maybe even less... usually after alot of jumping, basketball or recently snowboarding. Yesterday i was in the park hitting rails and doing jumps... i fell off a rail once with my leg straightened body prone(chest facing ground). My knee didn't hurt at all but knew that the position seemed vulnerable to injury. Snowboarded for the next two hours no pain at all. I'm home 5 hours later and feel a pain inside my knee when i put weight on it or straighten my knee. The next morning i wake up, no problems at all. This has happened in the past. Very very seldom, i can never get an answer of what it is. Snowboarding is a new activity im in my second year and i have been going twice a week since December. Never felt this before except for basketball.
You may want to try to strengthen your knee. Get an appointment with PT and have them evaluate you and give you exercises. At the moment, it doesn't sound like it's a huge problem for you and you want to keep it that way.