Meniscus injuries aren't always easy to know when they happen, I tore the medial meniscus in my left knee about 15 years ago and I had no idea when or how it happened either, one day I knelt down and felt a little something in my knee, figured it was a just a bruise or something minor since it didn't hurt a lot and the pain was minimal, it only hurt when I would kneel down or walk up a flight of stairs, most of the time I didn't even notice it, when it wouldn't go away I saw an orthopedist who ordered an MRI, that's when I found out. When you sprain or tear an ACL or MCL, you know right away something isn't right, but a meniscus is different, he could have done it running the bases or swinging a bat, there's no way to pin point when it happens since there's typically no immediate pain