The Phantom Walkman

So I had this walkman a while back (way back). It was a model WM-FX415, from the late 90s. I owned many walkmans (or walkmen?) before it, and this would happen to be the last one I would ever own. As time passed, I used it less and less, and eventually stopped. But I still carried it with me in my backpack out of habit. And being the obsessive type that I am, I kept replacing the batteries with new ones as each pair would discharge on its own from nonusage. This went on for years from late high school to early college; wasting my money on so many AA batteries. One day between classes, when I had nothing to do, I decided to take out my walkman to play with it. As I was replacing it's batteries, I noticed that the radio and clock would still work without batteries. Turning up the volume would make the radio fade away; turning it back down would return the phantom signal. The radio presets also worked. The clock would also change minute-to-minute without power. My walkman has never done this before. I kept playing with it for quite some time, and it kept running as if it was possessed. The phenomenon ended when I hit play on the tape player; the motor must have sucked whatever phantom juice was left in it. This was years ago, but I'm still puzzled by how this happened. Without an internal battery of some sort, it should not have been able to store any energy. What would explain this phenomenon?